reverse water gas shift reaction over supported cu-ni ... · reverse water gas shift reaction over...

Reverse Water Gas Shift Reaction over

Supported Cu-Ni Nanoparticle Catalysts

Maxime Lortie

Thesis submitted to the Faculty of Graduate and Postdoctoral Studies in

partial fulfillment of the requirements for the M.A.Sc degree

In

Department of Chemical and Biological Engineering

University of Ottawa

© Maxime Lortie, Ottawa, Canada, 2014

ii

Abstract

CuNi nanoparticles were synthesized using a new polyol synthesis method. Three

different CuxNi1-x catalysts were synthesized where x = 20, 50 and 80. The nanoparticles were

deposited on carbon, C, gamma-alumina, γ-Al2O3, yttria-stabilized zirconia, YSZ, and samarium-

doped ceria, SDC. Each set of catalysts was tested using the Reverse Water Gas Shift, RWGS,

reaction under atmospheric pressure and at temperatures ranging from 400°C-700°C. The

experiments were repeated 3 times to ensure stability and reproducibility. Platinum nanoparticles

were also deposited on the same supports and tested for the RWGS reaction at the same

conditions. The CuNi nanoparticles were characterized using a variety of different techniques. X-

ray diffraction, XRD, measurements demonstrate the presence of two CuNi solid solutions: one

Cu rich solid solution, and the other a Ni rich solid solution. X-ray photo electron spectroscopy,

XPS, measurements show Cu enrichment on all catalytic surfaces. Scanning electron

microscopy, SEM, measurements show CuNi nanoparticles ranging in size from 4 nm to 100 nm.

Some agglomeration was observed. SDC showed the best yield with all catalysts. Furthermore,

high oxygen vacancy content was shown to increase yield of CO for the RWGS reaction.

Cu50Ni50/SDC shows the combination of highest yield of CO and the best stability among CuNi

catalysts. It also has similar yields (39.8%) as Pt/SDC at 700°C, which achieved the equilibrium

yield at that temperature (43.9%). The catalyst was stable for 48 hours when exposed to high

temperatures (600-700°C). There was no CH4 observed during any of the experiments when the

partial pressure of the reactant gases was fed stoichiometrically. Partial pressure variation

experiments demonstrated the presence of CH4 when the partial pressure of hydrogen was

increased to twice the value of the partial pressure of CO2.

iii

Résumé

Des nanoparticules de CuNi ont été synthétisées à l'aide d'un nouveau procédé de

synthèse polyol. Trois différents catalyseurs de CuxNi1-x ont été synthétisés où x = 20, 50 et 80.

Les nanoparticules sont déposées sur : C, γ-Al2O3, YSZ et SDC. Chaque série de catalyseurs a

été testée en utilisant la réaction « Reverse Water Gas Shift », RWGS, sous pression

atmosphérique et à des températures allant de 400°C-700°C. Les expériences ont été répétées

trois fois pour assurer la stabilité et la reproductibilité. Des nanoparticules de platine ont été

également déposées sur les mêmes supports et testés pour la réaction RWGS dans les mêmes

conditions. Les nanoparticules ont été caractérisées en utilisant une variété de techniques

différentes. La diffraction par rayons-X, XRD, démontre deux alliages de CuNi: un alliage étant

riche en Cu et l'autre étant riche en Ni. La spectroscopie photoélectronique des rayons-X, XPS,

démontre un enrichissement en Cu sur toutes les surfaces catalytiques. La Microscopie

électronique à balayage, SEM, a démontré que les nanoparticules de CuNi ont une taille allant de

4 nm à 100 nm. L’agglomération de certaine particule a été observée. SDC a montré le meilleur

rendement parmi les supports avec tous les catalyseurs. La haute teneur en déficience d’oxygène

est suggérée d’avoir augmenté le rendement de CO pour la réaction RWGS. Cu50Ni50 / SDC

montre la combinaison de rendement de CO le plus élevé et la meilleure stabilité des catalyseurs

parmi CuNi. Ce catalyseur obtient des rendements similaires (39,8%) à Pt / SDC à 700 °C, qui a

atteint une conversion à l’équilibre à cette température (43,9%). Le catalyseur est stable pendant

48 heures consécutives lorsqu'il est exposé à des températures élevées (600-700 °C). Il n'y avait

pas de CH4 observé au cours des expériences lorsque la pression partielle des gaz réactifs est

alimentée de façon stoechiométrique. Des expériences variant la pression partielle des réactifs

démontre la formation du CH4 lorsque la pression partielle de H2 est deux fois celle du CO2.

iv

Acknowledgements

I would like to begin by thanking my supervisor; Marten Ternan. His countless hours

dedicated to my research gave me the opportunity to accomplish all I have worked towards over

the course of my Master’s degree. I am extremely grateful for his contributions to both my

professional career and my personal endeavours.

The Natural Science and Engineering Council (NSERC) and Phoenix Canada Oil

Company Limited, represented by Steve Aplin, are also acknowledged for their financial

contributions.

I would like to acknowledge the scientific contributions of several University of Ottawa

Researchers: Tara Burchell for her XRD assistance, Yun Liu for the SEM images, Elena

Baranova for the laboratory equipment and Alexander Mommers for the XPS experiments.

I would also like to thank the Chemical Engineering Technical Officers, Louis Tremblay,

Gérard Nina and Franco Ziroldo for their laboratory assistance throughout my degree.

Lastly, I would like to take this opportunity to thank my family and friends for their

endless support. I am truly blessed to have such an incredibly loving and caring group of

individuals willing to join me on my quest through life. I will never forget the contributions you

have brought me during the good and difficult times.

v

Table of Contents

Abstract ........................................................................................................................................... ii

Résumé .......................................................................................................................................... iii

Acknowledgements ........................................................................................................................ iv

Table of Contents .............................................................................................................................v

List of Figures ............................................................................................................................. viii

List of Tables ................................................................................................................................ xi

Nomenclature ................................................................................................................................ xii

Abbreviations ............................................................................................................................ xii

Symbols .................................................................................................................................... xiii

Chapter 1: Introduction ...................................................................................................................1

1.1 Motivation: .............................................................................................................................1

1.2 Objectives: ..............................................................................................................................5

1. 3 References: ............................................................................................................................7

Chapter 2. Literature Review ..........................................................................................................9

2.1 Nanoparticles: .........................................................................................................................9

2.2 Polyol Synthesis Method: .....................................................................................................10

2.3 Catalysts for the Reverse Water Gas Shift Reaction: ...........................................................13

2.3.1 Pt for the RWGS reactions ............................................................................................15

2.3.2 Cu RWGS reaction ........................................................................................................19

2.3.3 Doped Cu for the RWGS Reaction ...............................................................................21

2.3.4 Ni for RWGS Reaction ..................................................................................................24

2.3.5 Doped-Ni for the RWGS Reaction ................................................................................26

2.4 Support Interaction ..............................................................................................................27

2.5 References: ...........................................................................................................................30

Chapter 3 – Synthesis of CuNi/C and CuNi/γ-Al2O3 Catalysts for the Reverse Water Gas Shift

Reaction ........................................................................................................................................40

Abstract ......................................................................................................................................40

3.1 Introduction ..........................................................................................................................41

3.2 Experimental .......................................................................................................................45

3.2.1 Catalyst preparation ......................................................................................................45

vi

3.2.2 Physical Characterization .............................................................................................46

3.2.3 Reaction Experiments ...................................................................................................48

3.3 Results and Discussion ........................................................................................................49

3.4 Conclusion ...........................................................................................................................61

3.5 Acknowledgements .............................................................................................................62

3.6 References ...........................................................................................................................63

Chapter 4 – Synthesis of CuNi/YSZ and CuNi/SDC for the Reverse Water Gas Shift Reaction 65

Abstract .....................................................................................................................................65

4.1 Introduction .........................................................................................................................66

4.2 Experimental .......................................................................................................................69

4.2.1 Catalyst preparation ......................................................................................................69

4.2.2 Physical Characterization .............................................................................................70

4.2.3 Catalytic Performance ..................................................................................................71


4.3.1 Physical Characterization of Cu50Ni50 ...........................................................................73

4.3.2 Catalytic Performance ..................................................................................................74

4.3.3. Partial Pressure Variation and Stability Measurements ..............................................82

4.4 Summary and Conclusion ...................................................................................................86

4.5 Acknowledgements .............................................................................................................87

4.6 References ...........................................................................................................................87

Chapter 5 – General Discussion ....................................................................................................91

5.1 Introduction .........................................................................................................................91


5.3 Conclusion ...........................................................................................................................96

5.4 References ...........................................................................................................................96

Chapter 6: Conclusion ...................................................................................................................98

6.1 Summary of Results ............................................................................................................97

6.1.1 Objective 1: Synthesis of CuxNi1-x nanoparticles .........................................................98

6.1.2 Objective 2: Characterization of the CuxNi1-x catalysts ...............................................99

6.1.3 Objective 3: Supported Pt Nanoparticles for the RWGS Reaction ............................100

vii

6.1.4 Objective 4: CuxNi1-x Nanoparticles Deposited on Supports Having Varying Oxygen

Vacancy Contents for the RWGS Reaction .........................................................................101

6.2 Recommendations .............................................................................................................103

6.3 Contributions to Knowledge .............................................................................................105

6.4 References .........................................................................................................................108

Appendices ..................................................................................................................................108

A.1 Yttria-Stabilized Zirconia for the RWGS reaction using CuxNi1-x (x = 20, 50 and 80)

nanoparticles ........................................................................................................................108

A.2 Samarium-Doped Ceria for the RWGS reaction using CuxNi1-x nanoparticles ..........111

A.3 Conclusion .....................................................................................................................114

A.4 References .....................................................................................................................115

viii

List of Figures

Figure 1.1: Closed cycle using H2 electrolysis, RWGS and the Fischer Tropsch synthesis.......... 5

Figure 2.1: Model for the reaction mechanism of the RWGS reaction over Pt/CeO2..................18

Figure 3.1: XRD Spectra of colloidal: a) Cu20Ni80 b) Cu50Ni50 and c) Cu80Ni20 .......... ..............50

Figure 3.2: TEM image of Pt/C ...................................................................................................52

Figure 3.3: SEM image of a Cu50Ni50/C catalyst .........................................................................52

Figure 3.4: RWGS reaction at 1atm, PH2 = PCO2 = 1 kPa, balance He, GHSV = 176000 h–1

, 50

mg of catalyst: Cu80Ni20/C, 10 wt%...............................................................................................55


, 50

mg of catalyst: Cu50Ni50/C, 10 wt%..............................................................................................55


, 50

mg of catalyst: Cu20Ni80/C, 10 wt%...............................................................................................56

Figure 3.7: RWGS reaction at 1atm, PH2 = PCO2 = 1kPa, balance He, GHSV = 176000 h–1

, 50

mg of catalyst: Pt/C, 1 wt%...........................................................................................................56

Figure 3.8: RWGS reaction at 1atm, PH2 = PCO2 = 1kPa, balance He, GHSV = 282000 h–1

, 50

mg of catalyst: Cu80Ni20/γ-Al2O3, 10 wt%.....................................................................................57


, 50

mg of catalyst: Cu50Ni50/γ-Al2O3, 10 wt% ....................................................................................57


, 50

mg of catalyst: Cu20Ni80/ γ-Al2O3, 10 wt%....................................................................................57


, 50

mg of catalyst: Pt/ γ-Al2O3, 1 wt% ...............................................................................................58

Figure 3.12: Average yield of CO for the Reverse water gas shift reaction at 1atm, PH2 = PCO2 =

1 kPa, balance He, GHSV = 176000 h–1

, 50 mg of CuxNi1-x/C catalyst....................................... 59



, 50 mg of CuxNi1-x/γ-Al2O3 catalyst............................. 60

Figure 4.1: Schematic of experimental setup used for catalytic testing of the RWGS reaction...72

Figure 4.2: XRD of Cu50Ni50 colloidal solution ..........................................................................74

Figure 4.3: SEM of Cu50Ni50/C in a) LEI mode and b) COMPO mode ..................................... 74


, 50

mg of catalyst: Pt/YSZ, 1 wt% catalyst ........................................................................................75

ix


, 50

mg of catalyst: Cu50Ni50/YSZ, 10 wt% catalyst ............................................................................76


, 50

mg of catalyst: Pt/SDC, 1 wt% catalyst ........................................................................................77


, 50

mg of catalyst: Cu50Ni50/SDC, 10 wt% catalyst............................................................................78

Figure 4.8: Average yield of CO for the RWGS reaction using Pt supported on SDC and YSZ at

1 atm. Total flow rate of 510mL∙min-1

, PCO2 = PH2 = 1kPa, balance He.......................................78

Figure 4.9: Average yield of CO for the RWGS reaction using Cu50Ni50 supported on SDC and

YSZ at 1 atm. Total flow rate of 510 mL∙min-1

, PCO2 = PH2 = 1kPa, balance He..........................79

Figure 4.10: CO Yield versus bulk phase oxygen content in supports containing Cu50Ni50

nanoparticles at 700°C and 1 atm. Total flow rate of 510 mL∙min-1

, PCO2 = PH2 = 1kPa, balance

He ..................................................................................................................................................80

Figure 4.11: Diagram showing adsorbed species on the surface of a bifunctional catalyst

operating through an Eley-Rideal and a Langmuir Hinshelwood mechanism .............................83

Figure 4.12: Partial pressure variation using Cu50Ni50/SDC at varying temperatures. GHSV =

960800 h–1

. PCO2 = cst. Total flow rate of 510 mL∙min-1

...............................................................84


960800 h–1

. PH2 = cst. Total flow rate of 510 mL∙min-1

.................................................................85

Figure 4.14: Stability measurements of Cu50Ni50/SDC for 2 consecutive days of testing at a)

700°C and b) 600°C. GHSV = 960800 h–1

. Total flow rate of 510 mL∙min-1..............................86


700°C and b) 600°C. GHSV = 960800 h–1

. Total flow rate of 510 mL∙min-1

...............................93

Figure 5.2: CO yield versus bulk phase oxygen content in supports containing Cu50Ni50



He. Dashed line = equilibrium.......................................................................................................94


operating through an Eley-Rideal and a Langmuir Hinshelwood mechanism..............................95

Figure 5.5: Average yield of CO for the RWGS reaction using Cu50Ni50 and Pt supported on

SDC at 1 atm. Total flow rate of 510 mL∙min-1

, PCO2 = PH2 = 1kPa, balance He..........................96

Figure 6.1: Average yield of CO for the RWGS reaction using Pt supported on C, γ-Al2O3, SDC

and YSZ at 1 atm, PH2 = PCO2 = 1kPa, balance He with 50 mg of catalyst. Total flow rate of

mL∙min-1..................................................................................................................................... 101

x

Figure 6.2: Effect of oxygen vacancy on yield of CO for the RWGS reaction using Cu50Ni50

metal at 1 atm, PH2 = PCO2 = 1kPa, balance He, 50 mg of catalyst at 700°C. Total flow rate of

mL∙min-1. Dotted line = equilibrium yield..................................................................................102

Figure A.1: RWGS reaction at 1atm, PH2 = PCO2 = 1 kPa, balance He, GHSV = 716000h-1

, 50

mg of catalyst ……………………………...............................................................................109

Figure A.2 : RWGS reaction at 1atm, PH2 = PCO2 = 1 kPa, balance He, GHSV = 716000 h-1

over

a) Cu80Ni20/YSZ b) Cu50Ni50/YSZ c) Cu20Ni80/YSZ..................................................................110


, 50

mg of catalyst……………………………...................................................................................112

Figure A.4: RWGS reaction at 1atm, PH2 = PCO2 = 1 kPa, balance He, GHSV = 960800 h-1

over

a) Cu80Ni20/SDC b) Cu50Ni50/SDC c) Cu20Ni80/SDC..................................................................113

List of Tables

Table 2.1: The activation energies (kcal/mol) of steps in the forward and reverse directions in

the WGS reaction ..........................................................................................................................14

Table 2.2: Apparent activation energies and catalytic activities (at 300°C) of Pt over

supported........................................................................................................................................16

Table 2.3: Comparison of Pt and Cu over the same supports under similar testing

conditions.......................................................................................................................................19

Table 3.1: Carbon Catalyst physical characteristics .....................................................................47

Table 3.2: Cu-Ni surface ratios obtained from XPS measurements ............................................48

Table 6.1: CuxNi1-x / Carbon physical characteristics...................................................................99

Table A.1: Cu-Ni surface ratio measurements using XPS..........................................................111

xi

Nomenclature

Abbreviations

Abbreviation Definition

ACS American Chemical Society

ads Adsorbed

ALE Atomic layer epitaxy

BET Brunauer-Emmett-Teller

cat Catalyst

COMPO Compositional

EDS Energy dispersive X-ray spectroscope

EG Ethylene glycol

eg For example

EPOC Electrochemical promotion of catalysts

FESEM Field emission scanning electron microscope

GHSV Gas hourly space velocity

ICSD Inorganic Crystal Structure Database

IPPC Intergovernmental Panel on Climate Change

LEI Lower-secondary electron image

LTS Low temperature shift

Me Metal

NA Not available

NDIR Non-dispersive infrared

NSERC The Natural Science and Engineering Research Council

PEM Proton exchange membrane

PGM Platinum group metals

PVP Polyvinylpyrrolidone

RWGS Reverse water gas shift

SDC Samarium-doped ceria

SEM Scanning electron microscopy

Spec. Spectrometer

SOFC Solid oxide fuel cell

Sup Support

SV Space velocity

TEM Transmission electron microscopy

TPB Triple phase boundary

TOF Turn over frequencies

WGS Water gas shift

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

YSZ Yttria-stabilized Zirconia

xii

Symbols

Symbol Definition

a.u. Arbitrary unit

atm Atmosphere

°C Degrees Celsius

cm2 Centimeter square

° / deg Degree

Ea Activation energy

F Molar flow rate

g Gram

(g) Gas

Gt Giga tonne

h Hours

k Kelvin

kcal Kilo calorie

kg Kilogram

kPa Kilopascal

kV Kilovolt

KX Equilibrium constant of reaction x

L Liters

ma Milliamps

mg Milligram

min Minute

mL Milliliters

mmol Millimole

mol Mole

mW Milliwatt

m2 Square meters

N Order of reaction

nm Nanometers

O0X(S) Oxygen ion from support

Ppm Parts per million

Px Partial Pressure of X

V0X(S) Support vacancy

W Weight of catalyst

wt Weight

X Fractional conversion

[ ] Concentration

Me Conduction band of the metal

2ϴ Diffraction angle

% Percent

1

Chapter 1: Introduction

1.1 Motivation:

Carbon dioxide is the third most abundant gas in our atmosphere. Its concentration has

greatly increased since the industrial revolution. In the last 60 years, CO2 concentrations have

increased from 315 ppm to 400 ppm. According to the Intergovernmental Panel on Climate

Change (IPCC), 1000 ppm is attainable by year 2100. This near exponential increase can cause

drastic, irreversible changes to our planet’s climate.

Climate changes can be seen everywhere. Most recently, a large iceberg 6 times the size

of Manhattan broke off Antarctica plummeting into the ocean. Increase in CO2 concentrations

has also been linked to decreasing pH levels in the ocean. Corals such as the Great Barrier Reef

are feared to become extinct with the ongoing climate changes. Many believe that these

examples along with numerous others are directly linked to human activity.

Reducing CO2 emissions has been at the heart of countless research over the past 20

years. There is however a clear separation between researchers wanting to capture and store CO2

or use alternative energy sources, and others who want to reuse CO2 as a carbon source for the

production of chemicals/fuels. Some consider that using CO2 as carbon source brings negligible

contributions to the climate change [1].

Sizable quantities of CO2 are planned to be stored using carbon capture and sequestration.

It is estimated that by 2031, approximately 3 Gt/year of CO2 will be stored [2]. However, CO2

storage is not sustainable. Caverns that are used to store CO2 will one day be filled. The

technology can therefore be seen as a temporary fix over a concrete solution. The stored CO2

2

will consequently be available at zero cost. This means that large amounts of available CO2

could lead to new opportunities in CO2 utilization in the future.

CO2 is a thermodynamically stable molecule. Therefore, breaking the C=O bond requires

high energy. In order to do so industrially, high temperatures, hydrogen and catalysts are

required [3]–[5]. These demands are what limit the use of CO2 as an efficient carbon source in

the industrial field.

In contrast to fuel from crude oil, using fuel from CO2 is presently much more expensive

because of the high energy demands necessary to convert CO2 to fuel. This may not always be

the case. Current traditional extraction methods are slowly being replaced by offshore platforms

and oil sands which in turn cause an increase in the energy demand to produce crude oil. These

methods also produce high CO2 outputs consequently increasing CO2 concentrations. It is

feasible to imagine a future where fuel from recycled CO2 becomes less costly than fuel from

crude oil, because the high energy demands required for the recovery of crude oil limit the use of

crude oil thereby enabling renewable energy to become more competitive.

Several ways to sever the C=O bond exist. Electrocatalytic reduction of CO2 has been

achieved at room temperature and pressure using carbon supported Pt nanoparticles [6].

However, high electrical requirements limit the current use of the electrocatalytic reduction of

CO2. Other methods like, photochemical reduction [7], [8], electrochemical reduction [9], [10]

and the use of enzymes [11], [12] to reduce CO2 are small scale developing technologies which

limit their industrialisation potential.

Hydrogenating CO2 is an alternative capable of large scale industrialisation. Research has

been done converting CO2 directly to long-chain liquid hydrocarbons [13]–[15]. This research

3

combines two reaction steps into one process through a series of reactors and separation units.

The first reaction is the reverse water gas shift (RWGS) reaction (eq. 1.1) and the second is the

Fischer-Tropsch synthesis (eq. 1.2). Products capable of being synthesized through this reaction

include different types of fuels like gasoline, dimethyl ether - which is known as a possible

alternative to diesel - or light olefins containing up to 6 carbons (C6H12) [6].

CO2 + H2 ↔ CO + H2O (1.1)

(2n+1) H2 + n CO CnH(2n+2) + n H2O (1.2)

An alternative to the aforementioned process includes hydrogenating CO2 according to

the following synthesis reaction. Here, CO2 can be converted to methanol or higher molecular

weight alcohols.

xCO2 + (2x-z+y/2)H2 ↔ CxHyOz + (2x-z)H2O (1.3)

Without considering the end product, high operating temperatures required for the

endothermic RWGS reaction will undoubtedly require the use of a catalyst. Development of an

efficient catalyst capable of withstanding high temperatures and being selective to CO during the

RWGS reaction is a necessity if the process is to be used industrially. The process requires

standard equipment already available at most processing plants creating a good opportunity to

industrialize the hydrogenation of carbon dioxide through the RWGS reaction.

The selectivity of the catalyst is a requirement due to the nature of simultaneous side

reactions occurring during the hydrogenation process. Among possible species being formed,

methane is the most prominent. CH4 is an undesired by-product for several reasons. First, the

4

methanation process requires 4 H2 molecules as seen in eq. 1.4. In addition, natural gas prices

will always be relatively low in comparison to the cost of H2 manufactured from CH4.

4 H2 + CO2 CH4 + 2 H2O (1.4)

Currently hydrogen production is mainly accomplished via the steam reforming of

methane [1], [16] that is the reverse of the reaction in eq. 1.4. the entire basis of using the

RWGS reaction becomes ineffective when using steam reforming, because the production of

CO2 during this reaction is the opposite of the desired RWGS reaction. In addition, the process

uses hydrocarbons and heat.

Instead, H2 should be derived from alternate reaction pathways. The use of biomass,

biogas, cyanobacteria and green algae are all being researched with minimal productivity or

efficiency [1]. Using nuclear power or renewable energy like solar or wind power on the other

hand could be a profitable venture. This would be accomplished via the electrolysis of water.

Hydrogen was first believed to be a strong candidate to replace fuels in cars using proton

exchange membrane (PEM) fuel cells. The lack of flexibility to transport the gas coupled with

low energy density and an infrastructure already built for liquid hydrocarbons limit its uses as an

everyday gas. Converting H2 to hydrocarbons could effectively solve all of the aforementioned

problems since these fuels have higher energy densities, are convenient to transport and are

thoroughly established in today’s infrastructure.

Electrolysing water using both solar and wind energy is a topic already being thoroughly

researched [17]–[19]. Limitations are caused by high overpotential required for the process as

well as low overall efficiencies of solar and wind power [1]. Further research in this domain is

5

needed for the RWGS & Fischer-Tropsch process to become a useful and economical alternative

to other energy sources.

It is possible with the use of nuclear energy and renewable energy to create a closed cycle

loop for the fuel industry. As seen in Fig. 1.1, the cycle utilizes both products from the RWGS

reaction. Water which is generated during the process can be sent to the electrolysing unit to

make H2. CO2 generated from varying sources can be reduced to fuels and reused.

Figure 1.1: Closed cycle using H2 electrolysis, RWGS and the Fischer Tropsch synthesis.

Instead of being considered a “devil” molecule, CO2 could, with the proper initiative,

become a strategic molecule in the processing industry. Further research dedicated to converting

CO2 to useful fuels parallel to the current research devoted to capturing the gas can reduce CO2

emissions, and in the long term, reduce atmospheric CO2 concentrations.

1.2 Objectives:

The development of a stable and selective catalyst is required for the success of the

proposed strategy. This was considered to be attainable using platinum (Pt), copper (Cu) and/or

6

nickel (Ni) nanoparticles deposited on supports having varying oxygen vacancy content. With

that in mind, the following four objectives were pursued:

Objective 1: Develop a synthesis method producing CuNi nanoparticles

Objective 2: Characterize the CuNi catalysts

Objective 3: Establish a “best case scenario” using an established noble metal (Pt) based

nanocatalyst

Objective 4: Investigate the performance of CuNi nanoparticles deposited on supports

having varying oxygen vacancy contents for the RWGS reaction

The thesis is separated into 5 more chapters plus appendices. Chapter 2 provides a

detailed literature review pertinent to the research conducted during the Master’s project.

Chapters 3 and 4 are papers which are intended for submission for publication. Chapter 3

discusses the synthesis of the CuNi nanoparticles (Objective 1), their characterization (Objective

2) as well as how they compare to Pt (Objective 3) when deposited on supports containing no

oxygen vacancies. Chapter 4 further compares Cu50Ni50 to Pt (Objective 3) when both are

supported on supports having varying oxygen vacancy content (Objective 4). Chapter 5

integrates the material in the two articles (Chapters 3 and 4). Lastly, Chapter 6 summarizes the

major findings brought forth in this M.A.Sc thesis as well as further suggestions for prospective

researchers.

7

1. 3 References:

[1] G. Centi and S. Perathoner, “Chapter 1: Perspectives and State of the Art in Producing

Solar Fuels and Chemicals from CO2,” in Green Carbon Dioxide: Advances in CO2

Utilization, 2014, pp. 1–24.

[2] E. A. Quadrelli, G. Centi, J.-L. Duplan, and S. Perathoner, “Carbon dioxide recycling:

emerging large-scale technologies with industrial potential.,” ChemSusChem, vol. 4, pp.

1194–215, Sep. 2011.

[3] L. Wang, S. Zhang, and Y. Liu, “Reverse water gas shift reaction over Co-precipitated Ni-

CeO2 catalysts,” J. Rare Earths, vol. 26, pp. 66–70, 2008.

[4] C.-H. Huang, “A Review: CO2 Utilization,” Aerosol Air Qual. Res., pp. 480–499, 2014.

[5] W. Wang, S. Wang, X. Ma, and J. Gong, “Recent advances in catalytic hydrogenation of

carbon dioxide.,” Chem. Soc. Rev., vol. 40, no. 7, pp. 3703–27, Jul. 2011.

[6] B. Hu and S. L. Suib, “Chapter 3: Synthesis of Useful Compounds from CO2,” in Green

Carbon Dioxide: Advances in CO2 Utilization, 2014, pp. 51–97.

[7] J. Hawecker, J. Lehn, and R. Ziessel, “Photochemical and Electrochemical Reductions of

Carbon Dioxide to Carbon Monoxide Mediated by (2,2’-

Bipyridine)tricarbonylchlorohenium (I) and Related Complexes as Homogeneous

Catalysts,” Helv. Chim. Acta, vol. 69, no. 1986, pp. 1990–2012, 1990.

[8] M. Anpo, H. Yamashita, K. Ikeue, Y. Fujii, S. G. Zhang, Y. Ichihashi, D. R. Park, Y.

Suzuki, K. Koyano, and T. Tatsumi, “Photocatalytic reduction of CO2 with H2O on Ti-

MCM-41 and Ti-MCM-48 mesoporous zeolite catalysts,” Catal. Today, vol. 44, no. 1–4,

pp. 327–332, Sep. 1998.

[9] K. Hara, A. Kudo, and T. Sakata, “Electrochemical reduction of carbon dioxide under

high pressure on various electrodes in an aqueous electrolyte,” J. Electroanal. Chem., vol.

391, no. 1–2, pp. 141–147, Jul. 1995.

[10] M. Jitaru, “Electrochemical Carbon Dioxide Reduction - Fundamental and Applied

Topics,” J. Univ. Chem. Technol. Metall., vol. 42, no. 4, pp. 333–344, 2007.

[11] I. H. Steen, M. S. Madsen, N.-K. Birkeland, and T. Lien, “Purification and

characterization of a monomeric isocitrate dehydrogenase from the sulfate-reducing

bacterium Desulfobacter vibrioformis and demonstration of the presence of a monomeric

enzyme in other bacteria,” FEMS Microbiol. Lett., vol. 160, pp. 75–79, 1998.

[12] V. Wiwanitkit, “Plasmodium and host carbonic anhydrase : molecular function and

biological process,” Gene Ther. Mol. Biol., vol. 10, pp. 251–254, 2006.

8

[13] A. Boisen, T. V. W. Janssens, N. Schumacher, I. Chorkendorff, and S. Dahl, “Support

effects and catalytic trends for water gas shift activity of transition metals,” J. Mol. Catal.

A Chem., vol. 315, no. 2, pp. 163–170, Jan. 2010.

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sensitivity of the water-gas shift reaction over Cu-Zn-Al mixed oxide catalysts,” Appl.

Catal. A Gen., vol. 131, pp. 283–296, 1995.

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shift reaction on palladium / alumina,” Catal. Letters, vol. 28, pp. 313–319, 1994.

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hydrogen-permeable membrane reactor,” Appl. Catal., vol. 67, pp. 223–230, 1991.

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9

Chapter 2. Literature Review

2.1 Nanoparticles

Nanoparticles are now at the heart of a variety of different researches. In the past 20

years, these particles which are invisible to the naked eye have transformed how people look at a

variety of subjects. These subjects include electronics [1], optical [1], magnetic devices such as

sensors [2], photocatalysts [3], heterogeneous catalysis [4] etc.

Nanoparticles in heterogeneous catalysis have been increasingly used over the past two

decades [5]. In fact, there has been a near exponential growth of nanoparticle publications in a

13 year span from 1997 - 2010 [6]. This is in part because certain metals used in heterogeneous

catalysis are inactive/less active unless they are shrunk to nano-scale consequently changing their

properties. In addition, the small nature of these particles increases the number of surface sites

available for reaction.

Gold nanoparticles are a good example of changing properties once particle size is

reduced to nano-scale. For gold catalysts, there is a direct correlation between the particle size

and catalytic efficiency [7]. These gold nanoparticles can be synthesized with a narrow size

distribution and be an efficient catalysts while using a fraction of the metal [7]–[9].

Particle size distribution as well as particle shape has a lot to do with the synthesis

method used to produce nanoparticles. Laser pulses have been used to synthesize nanoparticles

of narrow size distribution capable of layering nanoparticles on certain surfaces resulting in a

specific thickness [9], [10]. Other methods use a combination of solvents and microwaves to

10

produce nanoparticles [11], [12]. However, both methods require high costs and produce

relatively small samples.

2.2 Polyol Synthesis Method

The polyol synthesis method is a (poly)ethylene glycol (EG) based method which utilizes

EG as both a solvent and a reducing agent. The method usually involves diluting metal salts in

EG and then either refluxing or distilling the solution at temperatures ranging from 140°C-196°C

[13]–[15].

Modifications can be made to the polyol synthesis method which can enhance the particle

size distribution and vary the shape of the resulting particles. These modifications can be

reaction conditions, solvent or salt based or can be through the addition of other species such as

stabilizing agents or pH adjustors.

Carroll et al. [16] synthesized copper (Cu) and nickel (Ni) nanoparticles using a polyol

method and varied several aspects. First, they used two reaction conditions: distillation and

reflux. Next, the scientists changed the solvent using ethylene glycol, diethylene glycol or

polyethylene glycol. Lastly, they used three different types of metal salts (chlorides, hydroxides

and acetates).

In all experiments, they dissolved their metal salts in an ethylene glycol solvent and

added NaOH as pH adjuster. A 0.1M metal salt concentration was used in 50 mL of the polyol

solvent. The mixture was then heated either through distillation or through reflux for 2 hours.

The particles were washed and centrifuged at 25°C.

Varying both solvents and reaction conditions brought shape and size variations. The

group noticed circular particles when ethylene glycol was used under reflux and pentagon shapes

11

when the same solvent was used under distillation. Similar changes were observed when solvents

were changed. Slightly smaller triangular particles were observed when using polyethylene

glycol under reflux while larger rectangular shaped particles were obtained with diethylene

glycol under reflux.

Varying salt precursors showed an effect on the reduction mechanism and resulting

metals. When chlorides were used in ethylene glycol under reflux, they observed the presence of

a Ni hydroxyl salt. When other salts were used, only base metals were obtained.

Other researchers [13] have also shown that varying metal salt precursors can affect how

said salts are dissolved in the polyol solvent. Bonet et al. found that Ni and Cu carbonates did not

dissolve completely in an ethylene glycol solution even after 39 hours of mixing at 140°C. They

noticed that nitrates salts were more soluble in the same solution.

The group first dissolved Cu and Ni nitrate in ethylene glycol at room temperature

without the addition of NaOH. They then refluxed the solution at 196°C. The reflux lasted for 4

hours. They observed the formation of a Ni rich Cu-Ni solid solution shell and a Cu rich Cu-Ni

solid solution core. This observation contradicts some research which state that Cu is the main

component found on the surface of a CuNi alloy [17]–[19].

Stabilizing agents are also used in the polyol process in order to form nanoparticles of a

narrow size distribution. Polyvinylpyrrolidone (PVP) has been widely used as stabilizing agent

[20], [21]. It is added prior to the synthesis. The polymer attaches to the formed metal preventing

agglomeration between nanoparticles. The PVP can then be burnt off prior to usage.

Meshesha et al. [22] compared the use of PVP with 1-hexadecylamine as stabilizing

agent in the synthesis of Cu nanoparticles. In their work, they found that 1-hexadecylamine was

12

more effective at isolating nanoparticles. The compound successfully capped the synthesized

metal and yielded smaller nanoparticles with a smaller size distribution.

Other researchers [20], [23]–[25] have found that PVP can be used to successfully

synthesize nanoparticles. Gold, silver, platinum, ruthenium among others are examples of noble

metal nanoparticles synthesized with a narrow size distribution using PVP.

NaOH can also be used as additive in the polyol process. Bock et al. [26] varied the

NaOH concentration from 0.2 to 0.4 M for the synthesis of PtRu nanoparticles using the polyol

process. They observed a relationship between the concentration / solution’s initial pH and

particle size. The higher pH/NaOH concentration yielded smaller nanoparticles. Other

researchers [27] noticed similar trends. Isaifan et al. [27] varied NaOH concentration of their Pt

nanoparticles and observed its effect on CO oxidation. They found that smaller particles yielded

better turn over frequencies (TOF).

The polyol process is also used to form bimetallic metals. In fact, some metals do not

begin to reduce to their metallic form without the presence of a nucleating agent. Ni is a good

example of such metal. Chou et al. [28] synthesized Ni99Pd1 nanoparticles using Pd as a

nucleating agent. The obtained metal was a bimetallic compound where Ni and Pd were pure

metals.

Other researchers have used the polyol process to synthesize core shell particles.

Nagaveni et al. [15] synthesized Pd coated Ni nanoparticles using the polyol process. They

injected Pd at 196°C in order to coat Ni particles with Pd. In addition, alloys can be synthesized.

Nanoparticles of FePt were synthesized by Liu et al [29].

13

In summary, the Polyol synthesis method provides a simple synthesis method capable of

generating nanoparticles of a narrow size distribution and of selected shape. Its flexibility

permits numerous extensions of the method to be developed that are capable of synthesizing a

variety of different metals.

2.3 Catalysts for the Reverse Water Gas Shift Reaction

Traditionally, catalysts were tested in an arbitrary manner but recently, screening

methods are employed to select catalysts with the most potential. These methods can involve

simulation software or experimental results. Because of this, catalyst research is becoming more

systematic.

Predicting catalytic activity can be a difficult task and therefore requires a certain level of

organisation. Zigarnik et al [30] demonstrated their methodology in which a computer program

first determines a list of intermediate species for reactions. Then they are able to calculate the

activation energies of each step. Table 2.1 demonstrates the activation energies of both directions

of the water gas shift reaction (Eq. 2.1). The crystal (111) face of 7 different metals was

examined. They determined that the metals should yield the following order of catalytic activity

based on kinetic simulations: Cu > Ni > Fe > Pt > Pd > Ag > Au.

H2O + CO CO2 + H2 (2.1)

14

Table 2.1: The activation energies (kcal/mol) of steps in the forward and reverse directions in

the WGS reaction [30]

Step

Metal

Cu(111) Ag(111) Au(111) Ni(111) Pd(111) Pt(111) Fe(110)

For Rev For Rev For Rev For Rev For Rev For Rev For Rev

H2Og = H2Oads 0 13.5 0 8.6 0 7.6 0 16.4 0 10 0 9.6 0 19

COg = COads 0 12 0 6.5 0 7 0 27 0 34 0 32 0 36

CO2, g = CO2, ads 0 5.3 0 3.2 0 2.8 0 6.5 0 3.8 0 3.6 0 7.7

H2, ads = Hads + Hads 12.7 15.3 15.4 10.6 19.4 3.6 8.2 23.3 8.8 22.2 9.5 21 6.2 26.8

H2, g = H2, ads 0 5.4 0 4.7 0 3.7 0 6.8 0 6.6 0 6.4 0 7.4

H2Oads = Hads + OHads 25.8 1.1 40.4 0 48.8 0 21.2 9.8 26.8 0 28.9 0 18.4 15.3

CO2, ads = COads + Oads 28 10.7 49.7 6 54.2 6.4 13.4 21.9 34.1 24.5 36.9 23.2 1.6 28

HCOOads = COads + OHads 20.4 0 22.3 0 21.2 0 13.2 5.5 7.1 11.3 7.5 10 12.2 11.5

OHads = Oads + Hads 15.5 20.8 18.3 13.2 20.6 7.9 12.8 27.9 14.6 21.6 15.1 20.4 11.5 31.7

CO2, ads + Hads = COads + OHads 22.5 0 38.5 0 35.1 0 12.6 6.1 17.5 0.9 19 0 8.7 14.9

HCOOads = CO2, ads + H,ads 1.4 3.5 0 16.2 0 13.8 3.5 2.4 0 20.8 0 21.5 6.8 0

CO2,ads + OHads = HCOOads + Oads 17.2 20.4 23.8 2.4 26.6 0 13.8 29.9 21.7 7.9 22.4 6.3 11 38

H2Oads + Oads = OHads + OHads 27.3 0 35.7 0 33.1 0 26.7 0 40.2 0 40.5 0 23.7 0

Oads + H2, ads = OHads + Hads 14.8 12.1 10.3 10.7 10.9 7.9 15.4 15.6 9 15.4 8.7 14.9 16.6 17.1

CO2, ads + H2Oads = HCOOads + OHads 27.2 0.4 56.6 0 62.7 0 21.6 11.1 47.6 0 50.4 0 16.7 20.4

CO2, ads + H2, ads = HCOOads + Hads 14.2 14.6 21.6 0.7 29.6 0 8.5 24.8 16.7 9.3 17.6 7.7 4.4 31.9

Another way of predicting the efficiency of a catalyst for a specific reaction is to observe

how it does in other similar reactions. For instance, supported noble metals like Pt, Rh, Ru etc

are known as a good hydrogenation catalysts because of their capability to dissociate hydrogen

[31]. They could therefore show good activity for the reverse water gas shift (RWGS) reaction.

In fact, several have tested Pt group metals (PGM) for the RWGS reaction and have shown

considerable conversion [32]–[34]. Supported Pt will be discussed in detail in section 2.3.1.

It is well known that catalysts that are good for a forward reaction should also be good

for the reverse reaction. The water gas shift (WGS) reaction has been thoroughly researched for

decades. Several catalysts have been established as being efficient depending on the operating

temperature. For instance, low temperature (<350°C) WGS reactions often involve Cu-ZnO-

15

Al2O3 catalysts while high temperature (>350°C) WGS reactions take place over Fe2O3-Cr2O3

catalysts [35].

This section is dedicated to highlighting the most pertinent advances in catalysis relevant

to the field of research. These include catalysts containing Pt, Cu and Ni. A section will also be

dedicated to the impact of the support. Other PGM metals such as Ru [36], Rh [37], Pd [38] and

noble metals such as Ag [39] and Au [40] have all been used for the RWGS reaction but will not

be discussed in detail unless it is directly related to the field of study.

2.3.1 Pt for the RWGS reactions

Pt has been used as a catalyst in numerous applications. These include in the catalytic

converter of your car, catalytic reforming of naphtha to higher octane gasoline, PEM fuel cell

technology, amongst others. However, platinum’s high cost limits its use as an effective

industrial catalyst.

Research into synthesizing Pt nanoparticles reduces the economic impact that is attached

with Pt. Researchers have been able to successfully synthesize Pt nanoparticles of specific size

and deposit them on varying supports [26], [41]. Pt nanoparticles have been researched for other

reactions [23], [24], [27] and have proven to be efficient at relatively small loadings (1 wt%

metal, 99 wt% support).

Supported metallic Pt in the micron scale has been tested for the WGS reaction showing

promising results. Among these, Pt-ZrO2 [42], Pt-Fe2O3 [43], Pt-CeO2 [44] and Pt–TiO2 [45]

have all shown considerable yields within the 250-400°C temperature range [35]. These

supported catalysts are compared in Table 2.2 which demonstrates the activation energy and the

16

catalytic activity. Pt/TiO2 demonstrates the best catalytic activity at 300°C and Pt/Fe2O3 shows

the worst. In the work of Ratnasamy and Wagner, catalytic activity is defined as:

Activity (mmol/kgcat) = Fco x (Xco/Wcat) (2.2)

where the molar flow rate of CO is Fco in mol/s, the fractional conversion is Xco and the weight

of catalyst is Wcat in kg.

Table 2.2: Apparent activation energies and catalytic activities (at 300°C) of Pt over supported

reducible oxides [35]

Catalyst Ea (kJ/mol) Activity (mmol/kg cats)

2% Pt/CeO2 65 15 1.5% Pt/ZrO2 58 20 1.9% Pt/TiO2 23 39 1.5% Pt/FeO3 44 6

In the past 10 years, Pt nanoparticles have been tested for the RWGS reaction. These are

typically deposited on reducible supports like TiO2 and CeO2 with positive results. The size of

the supported Pt particles range from 1 nm to 400 nm depending on the researchers conducting

the experiments.

Kim et al [33] tested Pt/TiO2 at different H2:CO2 feed ratios in a temperature range of 300

to 600°C. They used a space velocity (SV) of 12 000 h-1

, 500 mg of catalyst and a 1 wt%

loading. Pt nanoparticles had an average size of 22 nm. They achieved near equilibrium

conversion for their temperature range with no CH4 production. They also observed an increase

in conversion with an increase in H2:CO2 feed ratio. However, CH4 was observed when the ratio

was increased to 2. They also observed limited to no deactivation for a period of 72 hours.

The same researchers [46] compared the support’s reducibility using Pt nanoparticles.

They compared the use of titania (TiO2) as a reducible support with gamma-alumina (γ-Al2O3)

17

using 500 mg of catalyst, a gaseous feed content of 21% CO2, 30% H2, balance of N2 and a total

flow rate of 100 mL min-1

. They observed a considerable increase in CO2 conversion when using

Pt/TiO2. They suggest that the support’s reducibility is the main cause of their observation.

A high pressure setup using Pt/CeO2 was built by Tidona et al [47] where they operated

at a pressure range of 200 - 950 bar and 450°C. They found that the increase in pressure yielded

higher conversions, as predicted thermodynamically. They observed an increase in conversion

with pressure. The researchers explained that higher fluid densities yielding higher CO2 molar

flow rates at the inlet of the reactor causes the increase in conversion. Results were below

equilibrium for all their system.

By-products are often formed when trying to conduct RWGS reaction experiments; some

of which may even be beneficial. For example, Dorner et al.[48] attempted to modify the

reaction conditions in order to form valuable hydrocarbons directly. They used Co-Pt/Al2O3 and

had H2:CO2 feed ratios of 1:1, 2:1 and 3:1. They were able to shift the equilibrium slightly away

from the methanation reaction - which is formed by the hydrogenation of CO – by lowering the

operating pressure and reducing the H2:CO2 ratio. However, they concluded that throughout

their experiments, the catalyst behaved like a hydrogenation catalyst and were unable to

primarily form longer chain hydrocarbons (C2-C4).

Goguet et al. [49]–[51] worked with Pt /CeO2 studying catalytic deactivation as well as

spectrokinetic investigation for the RWGS reaction. In their work, they observed that CO is the

main cause of low temperature deactivation. In addition, high H2:CO2 ratios combined with

extensive catalytic testing can cause an accumulation of coke on the catalytic surface. Their

spectrokinetic research concluded that formates are almost entirely spectator species in the

18

formation of CO. They also found that Pt bound carbonyls are not a major reaction route.

Instead, they concluded that support surface carbonyls are the main reaction intermediate. Figure

2.1 demonstrates their interpretation of the intermediates formed over a Pt/CeO2 catalyst. These

kinetic tests were performed at low temperatures (225°C) and high H2:CO2 ratios (4).

Figure 2.1: Model for the reaction mechanism of the RWGS reaction over Pt/CeO2. [49]

Pekridis et al. [34] were among the first and only researchers to test Pt/YSZ/Pt for its use

in a SOFC with the RWGS reaction. In their work, they operated at different H2:CO2 ratios in a

temperature range of 650 – 850 °C. They applied both positive and negative potentials in order to

examine the impact of migrating O2-

species throughout the support. They observed that with

positive over potentials, O2-

was being pumped to the surface increasing the formation of H2O

and decreasing the formation of CO. When a negative current is applied, O2-

is sent away from

the surface generating more oxygen vacancies consequently increasing the formation rate of CO.

They developed the following mechanism for the support interaction:

CO2 + Vox(S) CO + Oo

x(S) (2.3)

H2O + Vox(S) H2 + Oo

x(S) (2.4)

19

In their work, they determined that the formate decomposition describes best the kinetics for the

RWGS reaction. They also concluded that the reaction rate seems to be controlled by C-

containing intermediates. This is mainly caused by the intermediates’ interaction with adsorbed

hydrogen to form carbonyl species.

2.3.2 Cu RWGS reaction

Copper (Cu) provides a relatively inexpensive alternative to PGM catalysts. Cu has been

most prominently used in both steam reforming and in the WGS reaction. In fact, Cu has proven

to be an effective catalyst for low temperature systems like the ones mentioned previously.

Issues with Cu arise when operating temperatures increase consequently reducing its catalytic

activity [35].

Industrially, Cu is an established metal for low temperature WGS reactions [42]. It is

even known as the “... most popularly studied catalytic system for the WGS reaction.” [31] In

addition, ongoing research published in 2014 continues to be dedicated to enhancing the metal’s

capabilities for the reaction. Jeong et al. [52] synthesized a Cu/CeO2 catalyst showing 100%

selectivity towards CO2 and excellent conversion. Others [53], [54] have looked into the

interaction of the support (ZrO2) using Cu metal. Cu deposited on CeO2 and ZrO2 have similar

activation energy and activity then Pt when deposited on the same supports as seen in Table 2.3.

The most popular catalyst for the for low temperature shift (LTS) WGS reaction is CuO-ZnO-

Al2O3 [35].

Table 2.3: Comparison of Pt and Cu over the same supports under similar testing conditions

Catalyst Ea (kJ/mol) Activity (mmol/kg cats)

2% Pt/CeO2 [35] 65 15 1.5% Pt/ZrO2 [35] 58 20 2.1% Cu/CeO2 [35] 43 16 6.1% Cu/ZrO2 [54] 66.5 19.3

20

Historically, research has shown that if a catalyst is good in one direction of an

equilibrium reaction, it has the potential to be good in the opposite direction [55]. This principle

has brought many to explore the use of Cu in the RWGS reaction. A spike of interest for the

RWGS reaction over the past 20 years further increased research for the hydrogenation of CO2

over a Cu-based catalyst.

Early research concerning the catalyzed hydrogenation of CO2 over Cu metal catalysts

was done using ZnO and Al2O3 as a support primarily because of its impact on the WGS

reaction. Work began on kinetic experiments attempting to understand the mechanism over a Cu

based catalyst. The CuO/ZnO/Al2O3 catalyst often used in LTS WGS reaction was studied by

Ginés et al. [56] in an attempt to compare their calculated results with experimental results in

order to predict reaction rates. There was a good agreement between both sets of data when they

assumed that both the CO2 dissociation and the water formation determined the overall reaction

rate.

Fujita et al. [57] and Campbell et al. [58], [59] studied the RWGS reaction mechanism

over Cu/ZnO2. They both suggested that the mechanism proceeds by a surface oxidation of Cu to

CuO. H2 could then reduce CuO to metallic Cu forming H2O. Campbell et al. also postulated the

formation of formate species as a major intermediate.

Chen et al. [60]–[62] studied the kinetics of metallic Cu over Al2O3 and SiO2. They also

suggested the impact of formate species. They discuss that formates are intermediate species

caused by the association of adsorbed hydrogen atoms with CO2. In addition, they showed the

presence of Cu2O formed when oxygen adatoms react with Cu0.

21

A significant advantage to using Cu as a catalyst for the RWGS reaction is its tendency to

favour the formation of CO (Eq. 2.1) over side reactions such as CO methanation (Eq. 2.5) [63].

CO + 3H2 CH4 + H2O (2.5)

However, Cu alone has shown to sinter at high temperature because of its relatively low melting

point of 1084.6°C [64]–[66].

One method proposed by Chen et al. [67] to increase thermal stability was to do atomic

layer epitaxy (ALE) to prepare Cu/SiO2 nanoparticles. They showed increased thermal stability

of the Cu nanoparticles claiming that the catalyst had different characterization in contrast to

standard Cu. What happens in this case is that the Cu particles were prevented from contacting

each other. Additionally, they mention that the deposition method provides high catalytic

activity for the transformation of CO2 to CO. This is because sintering was prevented causing no

loss in copper surface area.

Promoting Cu with varying metals has also shown to increase thermal stability and in

some cases even conversion. Doping Cu has been done extensively for the WGS reaction.

Amongst others, ZnO, Fe2O3 and Cr2O3 have all been used to dope Cu providing an increase in

activity [35], [68]. The same principle is applied for the RWGS reaction as seen in the following

section.

2.3.3 Doped Cu for the RWGS Reaction

Doping Cu with more stable metals is a reasonable solution to improving the thermal

stability by changing the physical properties of the Cu-containing catalyst. The doping agent

could itself be a more stable metal capable of withstanding higher operating temperatures.

Hughes [69] suggested the following order for metallic thermal stability:

22

Ag < Cu < Au < Pd < Fe < Ni < Co < Pt < Rh < Ru < Ir < Os < Re

Iron was amongst the first Cu dopants successfully used in the RWGS reaction. The

reasoning behind utilizing Fe containing catalysts is taken from their increased thermal stability

as shown previously and its good activity towards the WGS reaction [70]–[72]. In addition, Fe

has a much higher melting point (1535°C) than Cu (1083°C) making it a good candidate for high

temperature reactions [66].

Chen et al. [65], [73] studied the effect of stabilizing Cu/SiO2 with an Fe promoter. In

their work, 10wt% Fe-Cu/SiO2 was used. In all, 9.2% of the mixture was Cu while 0.8% was Fe.

The iron addition proved effective. Their catalyst was stable for up to 120 hours at 600°C under

40 mL/min of reactants being fed through the catalyst. In contrast, Cu/SiO2 deactivated almost

entirely after the same duration of time when exposed to the same flow rates. The high stability

is explained by the formation of small Fe particles formed around Cu particles that inhibit

sintering. Otherwise, the Cu/SiO2 catalyst sinters reducing the active surface area. They also

noticed an increase in conversion of 6% with the Fe additive.

Stone et al. [74] researched the impact of doping Cu with ZnO for low temperature

RWGS reaction (240°C). Their experimental procedure consisted of varying the Cu:ZnO ratio

from Cu rich to ZnO rich in order to examine its impact on conversion. They observed an

increase in conversion at high Cu content and highest conversion when the Cu:ZnO ratio was >3.

No stability measurements were performed.

Another group [62] studied the effect of potassium on a Cu/SiO2 catalyst. Their

experiments consisted of depositing 9 wt% Cu on the support and adding between 0.52wt% - 5.2

wt% potassium. They noticed a decrease in surface area with increasing amounts of dopant.

23

However, 0.52 wt% and 1.9 wt% K showed higher conversion than Cu alone in spite of the

reduced surface area. An increase of 7% is noticed at 600°C with the 1.9 wt% K additive. Further

doping caused a significant reduction in conversion. No stability tests were conducted with any

of the doped catalysts.

High temperature RWGS experiments using a Cu-Ni bimetallic mixture has been

researched by one group [63]. The main topic of the paper was to examine the effect of the

Cu:Ni ratio on selectivity and CO2 conversion for the RWGS reaction. 20 wt% of metal was

deposited on γ-Al2O3 and tested at 500°C and 600°C. The Cu/(Cu+Ni) ratios used were 0.75,

0.50, 0.25 and 0.17. A gas hourly space velocity (GHSV) of 1000h-1

was used with the reactants

fed stoichiometrially according to the RWGS reaction. Their results are given in form of

conversion and selectivity shown in Eq 2.6 and 2.7 respectively.

CO2 conversion (%) = ( ) ( )

( ) x 100% (2.6)

CO selectivity (%) = ( )

( ) ( ) x 100% (2.7)

The researchers noticed a decrease in CO selectivity and conversion when the Cu/(Cu+Ni) ratio

was below 0.75. Conversion of 28.7% at 600°C was observed with CO selectivity of 79.7%

using that ratio. The conversion and selectivity drop to 27.1% and 71.8% respectively when the

ratio is decreased (higher Ni content). Consequently, the selectivity towards CH4 is increased.

The CO yield, calculated using Eq 2.8, is 19.5% at higher Ni loadings compared to 22.9% at

lower Ni loadings.

CO yield (%) = ( )

( ) x 100% (2.8)

24

The group concluded that high Ni contents yielded increased formation of CH4 while high Cu

content generated more CO.

2.3.4 Ni for RWGS Reaction

Nickel is another transition metal with good hydrogenation behaviour. Its stability at high

temperatures is similar to that of Fe with a melting point of 1455°C [66]. However Ni is known

for its capability to further hydrogenate CO to CH4 [31], [75]–[77] making it a lesser candidate

for the RWGS reaction. Hydrogenating CO is an undesired step because it inhibits the

application of the Fischer-Trospch process which utilizes a syngas mixture to make long chain

hydrocarbons or alcohols.

Nickel remains a candidate as a catalyst for the RWGS reaction despite its high CH4

selectivity. This is in large part caused by the high operating temperatures used in the RWGS

reaction. According to Gibbs equilibrium calculations, the CO methanation reaction is strongly

favourable at lower temperatures (below 350°C) [35]. At the high operating temperatures the

methanation reaction is somewhat less favourable which accounts for the use of Ni as a possible

catalyst for the RWGS reaction.

Still, a lot of research has gone into the methanation of CO2 using Ni as a catalyst [75]–

[80]. This reaction uses lower operating temperatures in the goal of making a closed loop carbon

cycle using renewable energy to make H2. High H2 requirements combined with natural gas’ low

cost would make for an unprofitable venture.

Other work consists of hydrogenating CO2 to produce formic acid and/or methanol. Peng

et al. [81] studied the mechanism over Ni(111) and Bermudez et al. [82] studied methanol

synthesis over a Ni/γ-Al2O3 catalyst. The latter used coke oven gas as a feed source of CO2.

25

Low temperature (270°C) RWGS reaction was done using Ni/C compared to Co/C by

Guerrero-Ruiz et al in the mid 1980’s [83]. Their work examined the conversion of CO2 into

either CH4 or CO. They found similar conversions and both formed CH4 and CO. However, they

found Ni to be slightly more selective to CO than Co at the low operating temperatures.

High temperature RWGS experiments using Ni/CeO2 were performed by Wang et al. in

order to examine the preparation method [84] and the metal loading [85] effect on the RWGS

reaction. They operated at high temperatures (400-750°C) and low reactant flow rates (50

mL/min) fed stoichiometrically according to the RWGS reaction. In their work, they found that

both the preparation method and the metal loading influenced the results for the RWGS reaction.

First, they noticed that the Ni particles formed deficiencies within ceria’s crystal lattice creating

oxygen vacancies which increased conversion. They also found that 2wt% loading had the best

conversion amongst a range of 0 to 20 wt%. Their catalyst showed high CO formation and some

CH4 yields.

Other groups [86], [87] studied the reaction kinetics on Ni providing in depth analyses of

the production of formates on Ni surfaces. The group claims that formate species are a “dead-end

spectator molecule.” As mentioned in section 2.3.1, Goguet et al. [49] also suggested that

formate species did not actively participate in the reaction. Instead, the reaction pathway would

go through a Eley-Rideal mechanism [86], [87].

A lot of research has gone into doping Ni in order to examine its effect on selectivity and

conversion. The following section is devoted specifically to that research.

26

2.3.5 Doped-Ni for the RWGS Reaction

The doping effect is often examined in the hopes of enhancing a certain negative aspect

of a metal without eliminating whatever advantage the metal had in the first place. Research on

dopants for Ni catalysts has been done either in hopes of increasing CO selectivity or in order to

examine reaction mechanisms on different surfaces.

Potassium (K) was amongst the first dopants used with Ni for the RWGS reaction.

Campbell et al. [88] looked into adding different weight loadings of K to a Ni/SiO2 catalyst. The

researchers operated at low temperatures (280°C) and increased H2:CO2 ratios (3.3:1). They

noticed an increase in the CO turnover number and a decrease in the CH4 turnover number with

loadings of up to 0.81wt% K. Their work showed that doping Ni with K can increase CO

selectivity.

Other researchers [89] further tested K-doped Ni deposited on Al2O3 for the RWGS

reaction at 500°C. They discovered the formation of coke on top of their Ni-K particles. In fact,

the coking mechanism was strongly dependant on the K loading. The resulting coke was in the

form of nanofibers. Results showed that the coking was a direct result of CO2 reduction to

carbon.

Rare earth metals such as Ce and La have also been used as doping agents for the RWGS

reaction using a Ni catalyst. Barrault et al. [79] tested different weight loadings of La and Ce

over carbon supported Ni. Their first tests had approximately 1 wt% of La and Ce over 5 wt% of

Ni. They noticed a dramatic increase in CH4 formation with the addition as well as overall

catalytic activity. CO selectivity was reduced from 58% (only Ni) to 1.5% with 1 wt% La.

Similarly, 19.5% selectivity towards CO was observed with 1 wt% Ce. Further doping increased

27

the selectivity towards CH4 above 95% for both dopants. These results are most likely caused by

the fact that CO2 adsorbs strongly on the surface of rare earth oxides [79].

As mentioned in section 2.3.2, a Cu-Ni bimetallic metal was tested for the RWGS

reaction [63]. Cu-Ni bimetallic compounds were also tested for the WGS reaction by Lin [6].

The researcher noticed increased activity towards the WGS reaction when an equimassic ratio of

Cu-Ni was used when deposited on Al2O3.

2.4 Support Interaction

The interaction between metal catalyst and the support on which it is deposited has been

demonstrated for thousands of reactions and continues to be a corner stone in the catalysis world.

It is a well proven fact that catalysts generally perform better under a metal/support configuration

rather than strictly metal. In addition, aspects such as support reducibility [46], [90]–[92],

conductivity [93]–[95], surface area [96] and oxygen vacancies [97], [98] can impact how

catalytically active the support is and can even participate in the reaction.

Metal-support interaction using reducible supports is a well-known process in catalysis.

Gases can either be reduced or oxidized because the support is capable of exchanging ions with

adsorbed specie. This mechanism has been successfully proven for the WGS reaction [45], [64],

[92], [99] where the support can increase selectivity and conversion of the WGS reaction.

Support conductivity is a phenomenon developed by Vayenas and Stoukides [100] in the

early 1980’s when they applied a potential to their Yttria-stabilized Zirconia (YSZ) pellet. Ionic

conductivity has been exploited for the RWGS reaction by Pekridis et al. [34] as seen in section

2.3.1. This phenomenon was later proven without electrical current. Vernoux et al. [101] recently

28

claimed that the conductivity through the YSZ support can also be thermally induced without the

presence of an electrical current.

Oxygen vacancies can also actively participate in the WGS/RWGS reaction mechanisms.

Chen et al. [53] studied the impact of oxygen vacancies using different morphologies of ZrO2

with a Cu metal catalyst. They were able to synthesize ZrO2 having varying oxygen vacancy

contents and compare the different supports for the WGS reaction. They noticed an increase in

conversion with increasing oxygen vacancy content. Higher synergistic interaction was observed

between the Cu particles and the oxygen vacancies causing the increased catalytic activity.

Wang et al. [84] tested Ni-CeO2 varying the catalyst’s preparation method. Results

obtained showed that Ni was inserted within the crystal lattice generating oxygen vacancies. The

group concluded that those vacancies are the principle reason for the increased conversion that

they observed.

Daza et al. [102] recently examined the use of perovskite-type oxides for use in the

RWGS reaction. Their work consisted of synthesizing different perovskite-type oxides and

examining them for high temperature RWGS reaction. The catalysts had different oxygen

vacancy contents and were seen as oxygen carriers. This aspect permitted the oxides to interact

with gases providing increasing CO2 conversion.

The work conducted in this report was done using supports having varying oxygen

vacancy contents. Those supports were: gamma-alumina, γ-Al2O3, carbon black, C, yttria-

stabilized zirconium, YSZ, and samarium-doped ceria, SDC. Both C and γ-Al2O3 were

considered to contain no theoretical oxygen vacancies. YSZ and SDC have 0.074 and 0.1 oxygen

vacancies per cation respectively according to stoichiometry. Carbon black and gamma-alumina

29

have been thoroughly researched in both the WGS and RWGS reaction [31], [35], [82], [83],

[103]. However, YSZ and SDC have very limited research for their use in both reactions.

Firstly, alumina has been used extensively as a support for the WGS reaction [35]. It has

shown signs of deactivation at high temperatures when used for the RWGS reaction [65].

However, some have claimed that the support can be used extensively at high temperatures

(800°C) for long periods of time (50 hours) and experience no deactivation [82].

YSZ is best known for its use as a solid electrolyte for solid oxide fuel cells (SOFC)

[104]–[106]. It is also used as a ceramic insulation inside high temperature furnaces making it a

suitable support for high operating temperatures. However, YSZ has a relatively low surface

area, as seen in the experimental procedure section. YSZ must therefore depend on its oxygen

vacancies to provide increased catalytic activity.

Researchers have already shown that this support is capable of increased catalytic activity

for different reactions in comparison to both C and γ-Al2O3 [24], [41]. Few Researchers have

used YSZ as support for the RWGS reaction. As mentioned, Pekridis et al [34] was among the

first to use it in a SOFC using the RWGS reaction.

In addition, Ismail [107] studied the support alone without the addition of a metal catalyst

for the RWGS reaction. In his work, YSZ was exposed to different H2:CO2 ratios at temperatures

ranging from 650°C to 750°C. He noticed CO2 conversion at all conditions. YSZ had a 19%

conversion of CO2 at 700°C using a stoichiometric ratio of H2:CO2. The researcher does not

discuss whether the support is selective to CO or other by-products. The support’s activity

without the presence of a metal catalyst makes it a strong candidate for the RWGS reaction.

30

SDC is another support used for SOFC [108], [109]. Its high ionic conductivity and

thermal stability makes it a good candidate for its use as a solid electrolyte for SOFC. In

addition, SDC has more theoretical oxygen vacancies giving it potential to be a better support.

SDC is also among the least reducible ceria-doped powders available [108]. Therefore, the

impact of oxygen vacancies can be examined with less concern for the support reduction

mechanism.

Ismail [107] also researched SDC for its use as a support for the RWGS reaction. Like

YSZ, SDC was exposed to high operating temperatures (650-750°C) with different H2:CO2

ratios. He observed high CO2 conversions at all temperatures and H2:CO2 ratios. SDC obtained

30% conversion of CO2 at 700°C and a 1:1 H2:CO2 ratio.

Both YSZ and SDC have shown considerable CO2 conversion without the presence of a

metal catalyst for the RWGS reaction. Experiments detailed in this report will demonstrate the

impact these materials have on the RWGS reaction in comparison to more traditional supports

containing no theoretical oxygen vacancies.

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40

Chapter 3 – Synthesis of CuNi/C and CuNi/γ-Al2O3 Catalysts for the Reverse

Water Gas Shift Reaction

Abstract

A new polyol synthesis method is described in which CuNi nanoparticles of different Cu/Ni

atomic ratios were supported on both carbon and on gamma alumina and compared with Pt

catalysts using the reverse water gas shift, RWGS, reaction. All catalysts were highly selective

for CO formation. The concentration of CH4 was less than the detection limit. Cu was the most

abundant metal on the CuNi alloy surfaces, as determined by X-ray photo electron spectroscopy,

XPS, measurements. Only one CuNi alloy catalyst, Cu50Ni50/C, appeared to be as thermally

stable as the Pt/C catalysts. After three temperature cycles, from 400–700C, the CO yield at

700C obtained using the Cu50Ni50/C catalyst was comparable to that obtained using a Pt/C

catalyst.

41

3.1 Introduction

The carbon dioxide hydrogenation reaction has been proposed for use with carbon

capture technologies for the production of industrially viable chemicals, such as long chain

hydrocarbons, methanol, formic acid and carbon monoxide [1], [2]. When co-feeding CO2 and

H2 over a hydrogenation catalyst, there are two main hydrogenation processes that can take

place, the reverse water gas shift reaction

CO2 + H2 = CO + H2O (3.1)

and the subsequent hydrogenation of CO to either hydrocarbons or alcohols, depending on the

values of x, y, and z in Equation 3. 2.

x CO + (x-z+y/2) H2 = CXHYOZ + (x-z) H2O (3.2)

One of the reactions in Equation 3.2, the hydrogenation of CO to methane, is of particular

interest in this study.

CO + 2 H2 = CH4 + H2O (3.3)

When CO is selectively formed via Equation 3.1 and mixed with H2, the resulting syngas can be

a feed-stock for the Fischer Tropsch process [2] that produces liquid fuels. In contrast CH4

formed via Equation 3.3 is an undesirable by-product that is not convertible to liquid fuels in a

Fischer Tropsch process.

Wang et al. [1] have recently reviewed catalysts for the RWGS reaction. They reported

that noble metals have been studied and shown to be among the best catalysts for the RWGS

reaction because they generally promote H2 dissociation. Among noble metals used for the

RWGS reaction, platinum (Pt) has received considerable attention. It was found to produce high

CO yields [3]–[5]. In addition, Pekridis et al. [6] tested the electro-kinetics of the RWGS reaction

in solid oxide fuel cells containing a Pt/YSZ catalyst. The Pt/YSZ catalyst was found to be stable

42

at high temperatures and the cell achieved a maximum power density of 9 mW/cm2 [6]. In spite

of the recognized performance of noble metals as catalysts for the RWGS reaction, their main

drawback is their high cost that limits their commercial application.

Transition metals such as copper (Cu), nickel (Ni), and iron (Fe) are promising

alternatives to noble metal catalysts for the RWGS reaction. Both copper and nickel based

catalysts have shown good conversion for the WGS reaction as well as the RWGS reaction [7]–

[12]. Chen et al. [7] investigated Cu nanoparticles ranging in size from 2.4 – 3.4 nm and found

that the catalyst becomes unstable at higher temperatures [13]. These researchers [14] also added

Fe to Cu in an attempt to stabilize the catalyst. Although Fe alone had poor conversions, Fe

stabilized the Cu catalyst for 120 h and caused an increase in conversion of approximately 7% at

600°C. On the other hand, the Cu catalyst without the iron stabilizer deactivated rapidly and

reached zero conversion after 120 hours.

Similar research was performed by Chen et al [15] using Ni catalysts. Ni alone showed

high selectivity towards methane. When they added potassium to a Ni/ γ-Al2O3 catalyst they

reported higher selectivity towards CO even though they did not notice an increase in CO2

conversion. However with the potassium promoter they noticed the formation of coke.

Liu and Liu [16] studied a Ni-Cu catalyst that was prepared by immersing gamma-

alumina (γ-Al2O3) in an aqua ammonia solution of nickel nitrate and copper nitrate. Their

catalysts were not selective in that they reported large yields of both CH4 and CO. They

interpreted their results as CO2 being adsorbed on Cu and H2 being adsorbed on Ni.

The polyol synthesis method has been used extensively in the past for the synthesis of

metal particles from a metal salt precursor [Cu(NO3)2 or Ni(NO3)2 represented here as

43

Me(NO3)2]. Bonet et al. [17], [18] indicated that the overall reactions at 180C included the

following reactions to form acetic acid, glycolaldehyde, and glycolic acid:

(CH2OH)–(CH2OH) + Me(NO3)2 CH3COOH + 2 HNO3 + Me0 (3.4)

(CH2OH)–(CH2OH) + Me(NO3)2 (CH2OH)–(CHO) + 2 HNO3 + Me0 (3.5)

(CH2OH)–(CH2OH) + H2O + 2 Me(NO3)2

(CH2OH)–(COOH) + 4 HNO3 + 2 Me0 (3.6)

Bock et al. [19] found that oxalic acid, HOOC–COOH, is also formed and suggested that the

majority of the metal is formed by the oxidation of ethylene glycol to glycolic acid. At the

boiling point of ethylene glycol, 196C, Bonet et al. [17] indicated that the overall reactions

included the formation of diacetyl:

2 (CH2OH)–(CH2OH) + Me(NO3)2

(CH3CO)–(CH3CO) + 2 H2O + 2 HNO3 + Me0 (3.7)

where Me0 represents either copper or nickel in the metallic state. Bonet et al. [17] also

commented on the reaction mechanism and indicated that intermediate solid phases (metal

glycolates) precipitate before the metal powder is formed. Bock et al. [18] indicated that

oxidation products containing carboxylic acids act as stabilizers for metal colloid particles.

Specifically they indicated that glycolate anions, the deprotonated form of glycolic acid, are

good stabilizers for colloidal metal particles and that their concentration increases when the pH is

greater than 6. In their work, they increased the pH by the addition of NaOH and reported that

PtRu bimetallic nanoparticle sizes decreased when the pH was increased. Their explanation of

glycolate anions on the exterior of the metal particles preventing metal colloid agglomeration

seems to be consistent with their nanoparticle size results. The polyol synthesis method has

become known for its simplicity and accurate control of particle size [17], [18]. The addition of

44

NaOH during the polyol synthesis has been used to synthesize Pt nanoparticles of narrow size

distribution [20]–[23].

Other researchers [16], [18], [24] have also used the polyol synthesis method to obtain bi-

metallic particles. When alloying two metals together, the resulting reaction properties can often

be enhanced compared to the pure metal. In the past both Cu and Ni have been alloyed with

other metals to form alloys. For instance, Viau et al. [24] prepared Co-Ni and Fe-Ni particles

using the polyol synthesis method.

Bonet et al. [18] used a polyol synthesis method to obtain Cu-Ni particles. When nickel

carbonate and copper carbonate were used at 140°C they obtained a Cu-Ni powder composed of

both a Ni rich Cu-Ni solid solution and a Cu rich Cu-Ni solid solution. When the carbonates

were used at 196°C they obtained a Cu-Ni powder composed of a Cu rich Cu-Ni solid solution

and a solid Ni metal phase. They noted that the reduction temperature for Cu is less than for Ni.

Their particles had a particle size of 140 nm. They did not report any reaction results.

In this work base metal Cu-Ni nano-particle catalysts were prepared by a new synthesis

technique. Cu was chosen because it is selective for the formation of CO [16], although it is

unstable (sinters) at the higher temperatures where the equilibrium for the RWGS reaction is

more favourable. Ni was chosen because it also produces CO [15] although it also can form

unwanted by-products, CH4 and coke. One of the purposes of the investigations was to

determine if sintering of pure Cu could be prevented by the addition of Ni in the same way that it

was prevented by the addition of Fe [14]. The resulting CuNi catalysts are compared to results

obtained with Pt nano-particle catalysts that were synthesized and tested using a variety of

reactions: ethylene oxidation [21], CO oxidation [20] and toluene oxidation [23]. These Pt

catalysts are considered to be among the best catalysts for the RWGS reaction because they

45

achieve reaction equilibrium at some conditions. We report Cu-Ni catalyst compositions that

promote CO formation and inhibit CH4 formation at specific reaction conditions.

3.2 Experimental

3.2.1 Catalyst preparation

The synthesis of CuNi nanoparticles was achieved using a modified polyol technique.

First, 314.5 mg of nickel nitrate (Ni(NO3)2) (hexahydrate 99.999% metal basis, Alfa Aesar) was

dissolved in 30 mL of ethylene glycol (anhydrous 99.8%, SigmaAldrich) to obtain a green

solution. That solution’s pH was then increased to 11 via the addition of 199 mg of sodium

hydroxide (NaOH) pellets (EM Science, ACS grade) to obtain Solution 1. This caused the

solution to slightly darken. In a separate beaker, 321.8 mg of copper nitrate (Cu(NO3)2)

(hexahydrate 99.999% metal basis, Alfa Aesar) was dissolved in 30 mL of ethylene glycol to

obtain a blue solution. Its pH was also increased to 11 using 199 mg of NaOH pellets to obtain

Solution 2. Solution 2 also darkened. Following this, Solution 1 was poured into a round bottom

flask, refluxed and stirred at 196˚C. Once the temperature reached 196 ˚C, Solution 2, at room

temperature, was poured into the hot round bottom flask. The combined solution was refluxed at

196 ˚C for 30 minutes and then cooled. The combined solution gradually became dark brown in

colour. Once cooled, the colloidal particles were stored in the ethylene glycol solution. The final

pH of the combined solution was 7.

The colloidal particles were then deposited on supports, C and γ-Al2O3, using a wet

impregnation technique. A powdered support was placed into a beaker and subsequently an

amount of the combined solution was poured into the powder. The amount of combined solution

was chosen to result in 10 wt% CuNi and 90 wt% support. The solution/support was sonicated

for 1 hour and stirred for 24 hours. The supported metal was then centrifuged at 6000 rpm and

46

washed with 20 mL of deionized water 10 times to remove the salts remaining after the synthesis

procedure. The supports used were carbon black (Vulcan XC-72R, Cabot Corp. specific surface

area of 254 m2/g) and gamma-alumina (Alfa-Aesar, specific surface area 120 m

2/g). The catalyst

was then dried using a freeze dryer for 24 h. Prior to any experiments, the catalyst was crushed to

a fine powder using a mortar and pestle.

CuNi particles of three different compositions were prepared. The combined solution for

each composition was prepared with a different ratio of Solution 1 to Solution 2. The ratio was

selected to obtain CuNi colloidal particles of 80 wt % Cu / 20 wt % Ni (nominally Cu80Ni20), 50

wt % Cu / 50 wt % Ni (nominally Cu50Ni50), and 20 wt % Cu / 80 wt % Ni (nominally Cu20Ni80).

Pt nanoparticles were synthesized using a modified polyol method as described elsewhere

[20]. It involved diluting PtCl4 in a 0.06M NaOH solution of ethylene glycol. The mixture was

then refluxed at 160◦C for 3 hours. Once synthesized, the nanoparticles were deposited on carbon

black and gamma-alumina using the deposition technique described above. 1 wt% of Pt was

deposited on the support.

3.2.2 Physical Characterization

Transmission electron microscopy (TEM) of supported Pt nanoparticles was performed

using a JEOL JEM 2100F FETEM operating at 200 kV. Image J software was used to determine

the particle size distribution. Details of the Pt nanoparticle characterization have been described

elsewhere [20].

Scanning electron microscopy (SEM) of CuNi particles supported on carbon was

performed using a JEOL model JSM-7500F Field emission scanning electron microscope,

FESEM, in both lower-secondary electron image, LEI, and compositional, COMPO, modes set

at a distance of 8 mm with an acceleration voltage of 5 kV. An energy-dispersive X-ray

47

spectroscope (EDS) attached to the SEM machine was used to quantify the amount of Cu and Ni

found in the bulk CuNi particles.

X-ray diffraction (XRD) measurements were made on the CuNi colloidal particles using a

RigakuUltima IV difractometer which used a Cu Kα X-ray (40 ma, 44 kV) operating with

focused beam geometry and a divergence slit of 2/3 degree, a scan speed of 0.17 deg min-1

and a

scan step of 0.06 degrees were used while operating between 35° and 55°. The crystal sizes

determined by XRD are shown in Table 3.1.

Table 3.1: Carbon Catalyst physical characteristics Catalyst Metal

loading

(wt%)

XRD

Crystalline

size (nm)

Typical

particle

size (nm)

Pt / C 1 3.8

2.8

Pt / γ-Al2O3 N/A

Cu80Ni20 / C 10 30.2

64.4

Cu80Ni20 / γ-Al2O3 N/A

Cu50Ni50 / C 10 24

53.4


Cu20Ni80 / C 10 16.7

41.1


X-ray photoelectron spectroscopy (XPS) measurements were made on CuNi / C catalysts

using a KRATOS Axis Ultra DLD XPS in hybrid lens mode. The data were analyzed using the

XPS Peak program. The results are summarized in Table 3.2. XPS measurements were also

made on both Cu and Ni wires in order to compare the pure metal results to those for the CuNi

catalysts.

48

Table 3.2: Cu-Ni surface ratios obtained from XPS measurements

3.2.3 Reaction Experiments

The performances of the supported CuNi catalysts were evaluated using the RWGS

reaction. 50 mg of powdered catalyst was placed on a fritted quartz bed within a 35 mL quartz

tube to act as a fixed bed reactor. A gas mixture of 1 kPa H2 (Grade 4.0, Linde), 1 kPa CO2

(Grade 3.0, Linde) and the balance He (Grade 4.7 Linde) flowed through the reactor at a total

flow rate of 510 mL/min. The reaction was performed at atmospheric pressure using three

consecutive temperature cycles. Each temperature cycle consisted of a series of experiments

over the temperature range from 400°C – 700°C. Before each experiment, the temperature was

held constant for 30 min. Although the same mass of catalyst was used in each experiment, the

gas hourly space velocity (GHSV) was different because the supports had different bulk densities

(288 g/L for the carbon support and 461 g/L for the alumina support). The GHSV values were

176000 h–1

and 282000 h–1

respectively for CuNi/C and CuNi/Al2O3 catalysts. The effluent was

dehumidified by flowing through an adsorbent and was analyzed by flowing through a mass

spectrometer (Ametek Proline DM 100) and a non-dispersive infra-red CO gas analyzer (Horiba

VIA-510). Each set of experiments was repeated three times (24 hrs total) in order to examine

reproducibility and stability. The yield of CO was calculated using the following formula:

Yield of CO (%) = [CO]OUT / [CO2]IN x 100% (3.8)

Theoretical bulk

Cu/Ni ratio

from nominal

composition

Actual

Surface

Cu/Ni ratio

from XPS

Percentage

Increase

Cu20Ni80 0.25 1.2 380%

Cu50Ni50 1 2 100%

Cu80Ni20 4 4.2 5%

49

The mass spectrometer identified any by-products that were formed via side reactions

such as CO methanation. The mass spectrometer indicated the presence of gases with a

molecular weight of up to 50 atomic units and had a detection limit of 50 ppm.

3.3 Results and Discussion

The X-ray diffraction spectra of the CuNi nanoparticles are shown in Figure 3.1. The

positions of the peaks for both pure Cu (2 = 43.2, ICSD Collection Code: 53246) and pure Ni

(2 = 44.6, ICSD Collection Code: 43397) are shown as straight vertical lines in Figure 3.1.

They are the X-ray reflections from the 111 crystal lattice planes. The smaller peaks near 2

values of 50.4 and 51.5 are the reflections from the 200 crystal lattice planes of Cu and Ni

respectively. No species other than Cu and Ni were identified. There is a slight difference

between the peak positions for the pure metals and the metals in the catalysts. Deviations exist

because the catalysts are bimetallic solid solutions rather than pure metals. The catalyst that is

nominally Cu80Ni20 in Figure 3.1c had a 2 peak position of 43.345 that is slightly greater than

the one for pure Cu. The catalyst that is nominally C20Ni80 in Figure 3.1a had a 2 peak position

of 44.412 for Ni, that is slightly less than the one for pure Ni. This suggests that the two peaks

represent a Cu rich solid solution and a Ni rich solid solution.

50

Figure 3.1: XRD Spectra of colloidal: a) Cu20Ni80 b) Cu50Ni50 and c) Cu80Ni20

There is only one peak for the Cu80Ni20 catalyst in Figure 3.1c. That means that all of the

Ni was soluble in the Cu lattice. The single peak in Figure 3.1c is experimental evidence for a

copper rich CuNi solid solution in which the spacing between planes of the catalyst lattice is

close to that of pure copper.

There are two peaks for the Cu20Ni80 catalyst in Figure 3.1a. Because the first peak has a

2 value at 43.394, close to that for pure Cu, the spacing between its planes will be similar to

pure copper. Because the second peak has a 2 value at 44.412, close to that for pure Ni, the

spacing between its planes will be similar to pure nickel. Even though there is a large disparity

between the bulk Cu content and the bulk Ni content of the catalyst, the two peaks appear to

38 40 42 44 46 48 50 52 54

XR

D I

nte

nsity [

a.u

.]

2

a)

b)

c)

51

have similar areas. Therefore a substantial amount of Ni must be dissolved in the first Cu-like

peak, and the first peak must represent a CuNi solid solution. Since the second peak has a 2

value slightly different from pure Ni it will contain some Cu and that would make it a Ni rich

NiCu solid solution.

The observation of solid solutions is consistent with other work reported in the literature.

Bonet et al. [18] synthesized Cu-Ni particles using a similar technique. In their work they

refluxed copper and nickel carbonates starting materials in ethylene glycol. After 39 hours at

140°C they observed the presence of a copper-rich solid solution, Cu81Ni19, and a Ni rich solid

solution, Ni86Cu14.

The crystalline size of the synthesized nanoparticles increases with Cu content. A

summary of their diameters (15 – 65 nm) can be found in Table 3.1. They were calculated from

the XRD data using Scherrer’s formula. The CuNi particles described by Bonet et al. [17] had

diameters of 250 – 400 nm. It is possible that the longer refluxing times and the absence of

NaOH may have provided more opportunity for agglomeration.

TEM images of supported Pt / C nanoparticles were taken and a sample image is shown

in Figure 3.2. The particles are shown to be mainly spherical with a reasonably narrow size

distribution. The dispersion also appears to be relatively high. Numerical data derived from

several TEM images indicated that a typical particle size for the Pt/C particles was 2.8 nm (Table

3.1). Furthermore, the typical particle size from TEM analysis, 2.8 nm, is close to the bulk

crystalline size obtained from the XRD spectra, 3.8 nm.

52

Figure 3.2: TEM image of Pt/C

An SEM image for the Cu50Ni50/C catalyst is shown in Figure 3.3. The particles appear

to be generally spherical and to vary in size from 4 nm to 100 nm. This variation is mainly due to

visible signs of agglomeration.

Typical particle sizes for all of the CuNi catalysts measured by SEM are listed in Table

3.1. In general the CuNi particle sizes are an order of magnitude larger than the Pt particle size.

Since the CuNi metal loading, eg. 10 wt%, is an order of magnitude larger than the Pt loading,

eg.1 wt%, a greater extent of metal particle agglomeration might be expected for the CuNi

particles.

Figure 3.3: SEM image of a Cu50Ni50/C catalyst

An Energy-dispersive x-ray spectroscopy, EDS, analysis was also performed on the

catalyst. For example, the Cu80Ni20 / C catalyst had a measured composition of 82.7 wt% Cu and

53

17.3 wt% Ni. The EDS measurement was repeated at two different sites on the catalyst's surface

with reproducible results. The EDS results were consistent with the Cu80Ni20 nominal

composition of the synthesized particles. This indicates the synthesis method was successful in

obtaining the nominal Cu:Ni ratio that was intended.

XPS measurements were performed on the CuNi / C catalysts to determine their surface

compositions. The results in Table 3.2 show that the Cu surface concentration was greater than

that of the Cu bulk concentrations for all three of the CuNi / C catalysts. Furthermore, the

surface concentration of the Cu always exceeded the surface concentration of the Ni, even for the

nominal Cu20Ni80/C catalyst. That result is consistent with the literature. An early report by van

der Plank and Sachtler [25] indicated that Cu was the dominant species on the surface of CuNi

alloys. Subsequently Watanabee et al. [26] provided definitive experimental data for the

phenomenon. Later Sakurai et al. stated that the phenomenon had been conclusively shown [27].

In order to examine the physical changes of the catalyst, Cu50Ni50/C was examined by

SEM before and after exposure to high temperature and reactants. There were no visible signs of

additional agglomeration or any other physical changes to the metal in comparison to the

unreacted catalyst. In addition, an XPS analysis of the before and after catalysts was also

performed and showed no differences in the surface composition. This suggests that the catalyst

is compositionally stable at temperatures of at least 700°C.

The reverse water gas shift reaction was performed using eight different catalysts. These

results can be seen in Figures 3.4 – 3.11. In each figure the results for the 3 consecutive

temperature cycles, over the temperature range from 400°C to 700°C, are shown. Some of the

CuNi catalysts showed slight deactivation between the first and second cycle and also between

the second and third cycle.

54

The only observable components in the gas stream entering the mass spectrometer were

CO2, H2, CO, trace amounts of H2O and the carrier gas, He. These results indicate that CO was

the main product having a typical concentration of 2000 ppm. Other products including CH4 had

concentrations of less than the detection limit of the spectrometer, 50 ppm.

The absence of CH4 in the products was a highly desirable result, since CH4 is an

undesirable by-product if syngas for a Fischer Tropsch process is the goal. Cu is known to

favour CO production while CH4 is known to form on Ni catalysts [1]. Since some of the

catalysts used in this work contained 80 wt% Ni the absence of CH4 might be considered to be

inconsistent with the literature [1]. The advantage of the CuNi alloys made using this particular

polyol synthesis method is that more than one-half of the surface was composed of Cu, even for

catalysts having consisting of 80 % Ni in bulk metal, as was shown by our XPS results. Perhaps

the presence of sufficient Cu on the surface may allow CO to desorb before additional

hydrogenation occurs to form CH4.

The catalysts supported on carbon are shown in Figures 3.4 – 3.7. During the first

temperature cycle, the Cu80Ni20 metal carbon supported catalyst produced a slightly greater CO

yield than the Pt metal carbon supported catalyst. In addition, it was the only CuNi metal carbon

supported catalyst that produced a CO yield at 400°C during the first cycle. The yields of CO at

700°C using all the CuNi carbon supported catalysts differed from those using Pt by no more

than 3 %.

Deactivation was observed between the first and second temperature cycles for all of the

CuNi catalysts supported on carbon. Virtually no deactivation was observed when using the Pt

metal carbon supported catalyst. This suggests that the deactivation observed with the CuNi

carbon supported catalyst may have been related to the CuNi metal and not to the catalyst

55

support. The Cu50Ni50/C catalyst in Figure 3.5 was different from the Cu20Ni80/C and Cu80Ni20/C

catalysts in that no deactivation occurred between the second and third temperature cycles. This

suggests that after sufficient time-on-stream the performance of the Cu50Ni50/C may become

invariant with time, and that it may become a thermally stable catalyst.

Figure 3.4: RWGS reaction at 1atm, PH2 = PCO2 = 1 kPa, balance He, GHSV = 176000 h

–1, 50

mg of catalyst: Cu80Ni20/C, 10 wt% where = 1st cycle, = 2

nd cycle and = 3

rd cycle


–1, 50


nd cycle and = 3

rd cycle

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

56


–1, 50


nd cycle and = 3

rd cycle

Figure 3.7: RWGS reaction at 1atm, PH2 = PCO2 = 1kPa, balance He, GHSV = 176000 h

–1, 50

mg of catalyst: Pt/C, 1 wt% where = 1st cycle, = 2

nd cycle and = 3

rd cycle

The catalysts supported on gamma alumina, γ-Al2O3, are shown in Figures 3.8 – 3.11. In

general the CO yields on the γ-Al2O3 supported catalyst were slightly greater than those on the

carbon supported catalysts in Figures 3.4 – 3.7. For the first temperature cycle the CuNi metal γ-

Al2O3 supported catalysts produced CO yields at 700C that was 2 – 4 % less than those

produced by the Pt metal γ-Al2O3 supported catalyst. In addition, only the Pt metal γ-Al2O3

supported catalyst produced a non-zero CO yield at 400°C during the first cycle.

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

57

Figure 3.8: RWGS reaction at 1atm, PH2 = PCO2 = 1kPa, balance He, GHSV = 282000 h

–1, 50

mg of catalyst: Cu80Ni20/γ-Al2O3, 10 wt% where = 1st cycle, = 2

nd cycle and = 3

rd cycle


–1, 50

mg of catalyst: Cu50Ni50/γ-Al2O3, 10 wt% where = 1st cycle, = 2

nd cycle and = 3

rd cycle


–1, 50

mg of catalyst: Cu20Ni80/ γ-Al2O3, 10 wt% where = 1st cycle, = 2

nd cycle and = 3

rd cycle

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

58


–1, 50

mg of catalyst: Pt/ γ-Al2O3, 1 wt% where = 1st cycle, = 2

nd cycle and = 3

rd cycle

Deactivation was observed between the first and third temperature cycles for all the γ-

Al2O3 supported catalysts, both for CuNi and Pt. The smallest extent of deactivation, 2 %, was

observed with the Cu20Ni80 γ-Al2O3 supported catalyst. It is the CuNi catalyst with the smallest

Cu content. In contrast the CO yields at 700C for Cu80Ni20 and Pt γ-Al2O3 supported catalysts

decreased by over 8% between the first and third temperature cycles. In contrast, no deactivation

was observed for Pt metal carbon supported catalysts. This suggests that the γ-Al2O3 support

may contribute to the deactivation. It is known [28] that as the temperature is increased above

500C, gamma alumina can be converted to other phases such as delta alumina, theta alumina,

and alpha alumina. Alpha alumina has a much smaller surface area than gamma alumina. The

tendency of the γ-Al2O3 support to deactivate at high temperatures is consistent with literature

data [14], [29].

Liu and Liu [16] tested a similar catalyst having C50Ni50/γ-Al2O3 at a maximum

temperature of 600C. They used twice the metal content (20 wt%) and much smaller GHSVs

(much greater residence times in the reactor) and obtained CO2 conversions that exceeded the

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

59

ones being reported here. They used a catalyst preparation method that was different than the

one used here. Perhaps that may explain why the CH4 selectivity (eg. 28.2%) in their work was

much greater than in this work. Even though their CO2 conversions were greater than the ones

reported here, their finite selectivity to CH4 caused their CO yields to be similar to the ones

reported here. Liu and Liu [16] noticed an increase in CH4 when the Ni content of the CuNi

catalysts was increased.



, 50 mg of CuxNi1-x/C catalyst: The solid line represents

Cu20Ni80/C catalysts, the triangles represent Cu50Ni50/C catalysts, and the dashed line represents

Cu80Ni20/C catalysts.

A comparison of the average CO yields for the carbon supported catalysts are shown in

Figure 3.12. The CO yields obtained with Cu50Ni50/C catalyst are smaller than those obtained

with the other two CuNi catalysts. The Cu50Ni50/C catalyst was the one that appeared to be

compositionally stable according to the XPS results discussed previously. It was also the one

that appeared to be the most thermally stable in Figure 3.5. Perhaps there is an association

between minimum CO yield and thermal stability. In other words, perhaps the reaction sites on

CuNi catalysts with the largest turnover frequencies are the ones that are the most thermally

unstable.

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700 750

Yie

ld o

f C

O (

%)

Temperature (˚C)

60



, 50 mg of CuxNi1-x/γ-Al2O3 catalyst: The solid line

represents Cu20Ni80/ γ-Al2O3 catalysts, the triangles represent Cu50Ni50/ γ-Al2O3 catalysts, and

the dashed line represents Cu80Ni20/ γ-Al2O3 catalysts.

A comparison of the average CO yields for the alumina supported catalysts shown in

Figure 3.13 are almost identical regardless of catalyst composition. Nevertheless substantial

catalyst deactivation was evident in Figures 3.8–3.11. That suggests that the deactivation caused

by thermal transitions in alumina (loss of surface area, spinel formation) may have had more

influence on catalyst performance than variations in CuNi catalyst composition. In spite of the

similarity of the results in Figure 3.13, the CO yields for the Cu50Ni50/ γ-Al2O3 catalyst appear to

be slightly less than the other two catalyst compositions. That is consistent with the observation

in Figure 3.12 that the Cu50Ni50/C catalyst also produced smaller yields than the other two CuNi

catalysts.

As discussed above, some deactivation occurs when CuNi metal is supported on either

carbon or γ-Al2O3. As a result, most catalysts composed of CuNi metal supported on either

carbon or γ-Al2O3 will not satisfy one of the main objectives of this research, namely obtaining a

thermally stable catalyst capable of operating under high temperatures. The one exception

0

5

10

15

20

25

30

350 400 450 500 550 600 650 700 750

Yie

ld o

f C

O (

%)

Temperature (˚C)

61

discussed here is the Cu50Ni50/C catalyst. Unless deactivation can be mitigated CuNi catalysts

will not meet the requirements for an efficient, industrially viable catalyst.

3.4 Conclusion

The results of this investigation can be summarized by the following statements: The

new polyol synthesis method permits both Cu and Ni metal salts to be reduced at the same time

and at the same temperature, by first heating the Ni salt solution to 196C and then adding the Cu

salt solution that was at room temperature. In the past [18] a Ni rich surface was obtained

because Cu(NO3)2 was reduced first at temperatures as low as 140°C followed by Ni(NO3)2

reduction as the solution continued to be heated to 196°C. The CuNi alloy catalysts investigated

in this work are similar to pure Cu catalysts in that they show selectivity for CO formation and

the absence of CH4 formation. The selectivity to CO was attributed to Cu being the most

abundant metallic species on the surface of the catalyst, as determined by XPS measurements.

Although the CuNi alloys show some deactivation, they are not nearly as thermally unstable as

pure Cu (sintering) at the higher temperatures that are necessary for the equilibrium of the

RWGS reaction to be thermodynamically favorable. Deactivation was observed in each case that

an alumina catalyst support was used. With one carbon supported catalyst, Cu50Ni50/C, there

was little deactivation between the second and third temperature cycles and it appeared to be

compositionally stable. Finally, CO yields at 700C during the third temperature cycle with the

Cu50Ni50/C catalyst (16.6 %) were comparable to those with the Pt/C catalyst (16.3 %).

In conclusion, considering the difference in cost between CuNi alloys and Pt metal, these

results suggest more studies are warranted on the use of CuNi alloy catalysts for the RWGS

reaction.

62

3.5 Acknowledgements

The Natural Science and Engineering Research Council (NSERC) and Phoenix Canada

Oil Company Limited are gratefully acknowledged for their financial support.

3.6 References



[2] P. Vibhatavata, J.-M. Borgard, M. Tabarant, D. Bianchi, and C. Mansilla, “Chemical

recycling of carbon dioxide emissions from a cement plant into dimethyl ether, a case

study of an integrated process in France using a Reverse Water Gas Shift (RWGS) step,”

Int. J. Hydrogen Energy, vol. 38, no. 15, pp. 6397–6405, May 2013.



Apr. 2013.



pp. 100–107, May 2012.

[5] S. S. Kim, H. H. Lee, and S. C. Hong, “Applied Catalysis B : Environmental The effect of

the morphological characteristics of TiO2 supports on the reverse water – gas shift

reaction over Pt/TiO2 catalysts,” "Applied Catal. B, Environ., vol. 119–120, pp. 100–108,

2012.




vol. 127, no. 1–4, pp. 337–346, Sep. 2007.

[7] C. S. Chen, J. H. Wu, and T. W. Lai, “Carbon Dioxide Hydrogenation on Cu

Nanoparticles,” J. Phys. Chem. C, pp. 15021–15028, Aug. 2010.

[8] C. Chen, W. Cheng, and S. Lin, “Mechanism of CO formation in reverse water – gas shift

reaction over Cu/Al2O3 catalyst,” Catal. Letters, vol. 68, pp. 45–48, 2000.



63




[11] J. Lin, “Supported Copper, Nickel and Copper-Nickel nanoparticle Catalyst for Low

Temperature WGS reaction,” University of Cincinnati, 2012.






vol. 238, pp. 55–67, 2003.



106, Jan. 2004.






[17] L. Poul, N. Jouini, and F. Fie, “Layered Hydroxide Metal Acetates ( Metal ) Zinc , Cobalt

, and Nickel ): Elaboration via Hydrolysis in Polyol Medium and Comparative Study,”

Chem. Mater., vol. 12, no. 10, pp. 3123–3132, 2000.

[18] F. Bonet, S. Grugeon, L. Dupont, R. Herrera Urbina, C. Guéry, and J. M. Tarascon,

“Synthesis and characterization of bimetallic Ni–Cu particles,” J. Solid State Chem., vol.

172, no. 1, pp. 111–115, Apr. 2003.



Soc., vol. 126, pp. 8028–37, Jun. 2004.




43–57, 2011.



vol. 464–465, pp. 87–94, Aug. 2013.

64




E14, 2012.




pp. 996–1002, Jul. 2013.

[24] G. Viau, F. Fiévet, and F. Fiévet-Vincent, “Nucleation and Growth of Bimetallic CoNi

and FeNi Monodisperse Particles Prepared in Polyols,” Sol, vol. 84, pp. 259–270, 1996.





490, 1976.



[28] R. K. Oberlander, “Aluminas for Catalysts: Their preparation and properties,” Appl.

Industiral Catal., vol. 3, p. 67, 1984.

[29] J. Hu, K. P. Brooks, J. D. Holladay, D. T. Howe, and T. M. Simon, “Catalyst development

for microchannel reactors for martian in situ propellant production,” Catal. Today, vol.

125, no. 1–2, pp. 103–110, Jul. 2007.

65

Chapter 4 – Synthesis of CuNi/YSZ and CuNi/SDC for the Reverse Water

Gas Shift Reaction

Abstract

Cu50Ni50 nanoparticles were synthesized using a modified polyol method and deposited on

samarium-doped ceria, SDC, and yttria-stabilized zirconia, YSZ, supports to form reverse water-

gas shift, RWGS, catalysts. The best CO yields, obtained with the Cu50Ni50/SDC catalyst, were

about 90% of the equilibrium CO yields. In contrast CO yields using Pt/SDC catalysts were

equal to equilibrium CO yields at 700C. Catalyst selectivity to CO was 100% at hydrogen

partial pressures equal to CO2 partial pressures, 1 kPa, and decreased as methane was formed

when the hydrogen partial pressure was 2 kPa or greater. The reaction results were explained

using a combination of Eley-Rideal and Langmuir-Hinshelwood mechanisms that involved

adsorption on the metal surface and the concentration of oxygen vacancies in the support.

Finally the Cu50Ni50/SDC catalyst was found to be thermally stable for 48 hours at 600/700C.

66

4.1 Introduction

The emission of carbon dioxide into the environment is viewed by many as a major

contributor to global warming [1]. In spite of the current fossil fuel energy prices, the use of

fossil fuels continues to increase resulting in more carbon dioxide emissions. Those emissions

affect the state of the atmosphere and the state of the oceans.

Carbon capture and storage technologies are one means of diminishing CO2 emissions.

Unfortunately, storing CO2 in underground caverns is not sustainable because all the caverns will

eventually be filled. An alternative to carbon storage would be the reaction of CO2 with another

chemical to obtain a useful product.

This work describes the reaction of carbon dioxide with hydrogen obtained from a

renewable source. By hydrogenating CO2 [2], [3], it is possible to obtain syngas via Equation 4.1

or alcohols and hydrocarbons (using the Fischer-Tropsch process), via Equation 4.2.

CO2 + H2 ↔ CO + H2O (4.1)

x CO2 + (2x-z+y/2) H2 ↔ CxHyOz + (2x-z) H2O (4.2)

Equation 4.1 is the reverse water-gas shift reaction (RWGS). Equation 4.2 is a synthesis reaction

that is practiced on an industrial scale, the Fischer-Tropsch process. In Equation 4.2 when x = 1,

y = 4 and z = 0, methane (CH4) is formed. It is a common by-product of the RWGS reaction.

When x = 1, y = 4 and z = 1, the product is methanol (CH3OH). When z = 0 and 5 < x < 20, a

highly valued liquid hydrocarbon product is obtained.

An appropriate catalyst must satisfy several criteria. Because the equilibrium yield of CO

from the RWGS reaction increases with temperature, thermal stability of the catalyst is one

criterion. Others are rapid kinetics and selectivity to CO rather than to CH4.

67

Noble metals such as platinum (Pt) have been studied and proven to be among the best

RWGS catalysts because they have the ability to dissociate H2 [2]. Also, Pt is stable at high

temperatures and produces both high CO selectivity and CO2 conversion [4]–[6]. However,

noble metals such as Pt have a high cost.

Copper (Cu) and nickel (Ni) transition metals may be promising alternative catalysts to Pt

noble metals. One indication is that both of these metals have produced promising results with

the water gas shift, WGS, reaction [2], [7] that is shown in Equation 4.3.

CO + H2O ↔ CO2 + H2 (4.3)

Good results were also obtained when they were tested individually on catalyst supports [4]–[6]

using the RWGS reaction.

Metallic pure Cu catalysts have shown a tendency to deactivate over time when exposed

to high temperatures [8]–[10]. Stabilizers have been used in an attempt to improve copper’s

thermal stability. Chen et al [11] reported that an iron (Fe) additive stabilized Cu at 600°C for up

to 120 h. It also increased CO2 conversion by 7%. In contrast, Cu without Fe deactivated quickly

reaching 0% conversion after 120 h. Other additives include potassium [12] and zinc oxide [9].

Chen et al. [13] showed that a Ni catalyst used for the RWGS reaction enhanced the

formation of CH4 which is undesirable. They also investigated a Ni catalyst that was promoted

with a potassium (K) additive. Although K increased the selectivity toward CO, it also caused

the formation of coke.

Liu et al. [14] made a bimetallic Ni and Cu catalyst and used it for the RWGS reaction.

Their research was aimed at examining the effect of both metals on selectivity for the RWGS

reaction. They found that an increase in catalyst Ni content generated greater CH4 yields while

increased Cu contents generated greater CO selectivity.

68

Catalysts are often supported on materials such as γ-Al2O3, SiO2, and C. Yttria-stabilized

zirconia, YSZ, and samariam-doped ceria, SDC, are two less common supports that have shown

potential for the RWGS reaction [15], [16]. Both contain oxygen vacancies.

Yttria-stabilized zirconia, Y0.15Zr0.85O1.925, is a combination of 0.15* Y2O3 and 0.85 ZrO2.

It is a conductive ceramic which has been used in solid oxide fuel cells and sensor technologies

[17]. It is also used for the electrochemical promotion of catalysts, EPOC [18]. The purpose for

doping ZrO2 with Y2O3 is twofold: (1) Y2O3 stabilizes the cubic fluorite structure eliminating

volumetric variations caused by phase transformations, and (2) Y2O3 creates oxygen vacancies

within the ZrO2 lattice [19], [20]. Oxygen vacancies are important for all of the YSZ

applications mentioned above. In addition oxygen vacancies in ZrO2 have been shown to affect

the WGS reaction [7].

Similarly, adding dopants to ceria, CeO2, increases the oxygen vacancies within the

crystal lattice [8]. Doping with cations having an ionic radius and electronegativity close to CeO2

are considered to be the most appropriate [21]. Samarium showed the greatest resistance to

reduction of the CeO2 support [22] among the doping agents investigated. Since the intent of the

work was to examine the effect of oxygen vacancies and not the reducibility of the support,

samarium was chosen as the dopant. Sm0.2Ce0.8O1.9 (SDC) is a combination of 0.2*Sm2O3 and

0.8*CeO2.

YSZ and SDC have different oxygen vacancy contents. Samarium doped ceria,

Sm0.2Ce0.8O1.9, has 0.1 oxygen vacancies per cation when compared to CeO2. In contrast, yttria-

stabilized zirconia, Y0.15Zr0.85O1.926, has 0.074 oxygen vacancies per cation when compared to

ZrO2. Both of these materials have been tested by themselves without the addition of other

metals and have demonstrated thermal stability and CO2 conversion [15].

69

In the present work, Cu50Ni50 nanoparticles were synthesized using a modified polyol

technique and deposited on SDC and YSZ supports. The SDC and YSZ supported catalysts were

compared to catalysts containing Pt nanoparticles deposited on the same supports at the same

reaction conditions. The Cu50Ni50 catalyst with the best performance was examined at a variety

of partial pressures. In addition thermal stability experiments were performed in which the

temperature was maintained between 600 and 700°C for 48 h.

4.2 Experimental

4.2.1 Catalyst preparation

The synthesis of CuNi nanoparticles was achieved using a modified polyol technique.

First, 314.5 mg of nickel nitrate (Ni(NO3)2) (hexahydrate 99.999% metal basis, Alfa Aesar) was

dissolved in 30 mL of ethylene glycol (anhydrous 99.8%, SigmaAldrich) to obtain a green

solution. That solution’s pH was then increased to 11 via the addition of 199 mg of sodium

hydroxide (NaOH) pellets (EM Science, ACS grade) to obtain Solution 1. This caused the

solution to slightly darken. In a separate beaker, 321.8 mg of copper nitrate (Cu(NO3)2)

(hexahydrate 99.999% metal basis, Alfa Aesar) was dissolved in 30 mL of ethylene glycol to

obtain a blue solution. Its pH was also increased to 11 using 199 mg of NaOH pellets to obtain

Solution 2. Solution 2 also darkened. Following this, Solution 1 was poured into a round bottom

flask, refluxed and stirred at 196˚C. Once the temperature reached 196 ˚C, Solution 2, at room

temperature, was added to the round bottom flask. The ratio of Solution 1 to Solution 2 was

selected to obtain CuNi colloidal particles of 50 wt % Cu / 50 wt % Ni (nominally Cu50Ni50).

The combined solution was refluxed at 196 ˚C for 30 minutes and then cooled. The combined

solution gradually became dark brown in colour. Once cooled, the colloidal particles were stored

in the ethylene glycol solution. The final pH of the combined solution was 7.

70

The colloidal particles were then deposited on supports using a wet impregnation

technique. A powdered support was placed into a beaker and subsequently an amount of the

combined solution was poured unto the powder. The amount of combined solution was chosen

to result in nominally 10 wt% of CuNi when deposited on the supports, which corresponds to a

Cu50Ni50 metal/support ratio of 1 to 9. The solution/support was sonicated for 1 hour and stirred

for 24 hours. The supported metal was then centrifuged and washed with deionized water several

times to remove the salts remaining after the synthesis procedure. The supports were yttria-

stabilized zirconia (Tosoh, BET surface area 14.8 m2·g

-1) and samarium-doped ceria (FCM, BET

surface area 31.0 m2·g

-1). The catalyst was then dried using a freeze dryer. Prior to any

experiments, the catalyst was crushed.

Pt nanoparticles were synthesized using a polyol method as described elsewhere [23]. It

involved diluting PtCl4 in a 0.06 M NaOH solution of ethylene glycol. The mixture was then

refluxed at 160◦C for 3 hours. The deposition technique was the same as that for Cu50Ni50 and the

same supports were used. After deposition, each support contained 1 wt% of Pt nanoparticles.

4.2.2 Physical Characterization

X-ray diffraction (XRD) measurements were made on the CuNi colloidal particles using a

RigakuUltima IV difractometer which used a Cu Kα X-ray (40 ma, 44 kV) operating with

focused beam geometry and a divergence slit of 2/3 degree, a scan speed of 0.17 deg min-1

and a

scan step of 0.06 degrees were used while operating between 35° and 55°.

Characterization of the CuNi metal particles by scanning electron microscopy, SEM, and

X-ray photoelectron spectroscopy has been reported previously [24]. Characterization of the Pt

metal particles by transmission electron microscopy, TEM, has also been reported previously

[25]. The characterization measurements were made with the metal particles supported on

71

carbon, in order to avoid electrostatic charging that would occur when metals are supported on

insulating materials.

4.2.3 Catalytic Performance

Catalytic reaction rates were measured on both Cu50Ni50 and Pt supported nanoparticles

for the RWGS reaction. First, the dry metal/support catalyst was finely crushed. Then, 50 mg of

the metal/support catalyst was placed on top of a fritted quartz bed located within a tubular, 35

mL quartz reactor. A gaseous mixture containing 1 kPa H2 (Grade 4.0, Linde), 1 kPa CO2

(Grade 3.0, Linde) with the balance being He (Grade 4.7 Linde) flowed through the reactor at a

flow rate of 510 mL·min-1

. The reaction was performed at a total pressure of 1 atm using three

consecutive temperature cycles. Each temperature cycle consisted of a series of experiments

over the temperature range from 400°C – 700°C. Before each experiment, the temperature was

held constant for 30 min. The same mass of catalyst was used in each experiment, 50 mg, so that

the weight hourly space velocity was constant at 612 L/(h g). Nevertheless, the gas hourly space

velocity (GHSV) was different because the supports had different bulk densities (1187 g/L for

the YSZ support and 1570 g/L for the SDC support). The GHSV values were 716000 h–1

and

960800 h–1

respectively for CuNi/YSZ and CuNi/SDC catalysts. The effluent was dehumidified

by flowing through an adsorbent and was analyzed by flowing through a mass spectrometer

(Ametek Proline DM 100) and a non-dispersive infra-red CO gas analyzer (Horiba VIA-510).

Each set of experiments was repeated three times (24 hrs total) in order to examine

reproducibility and stability.

A schematic of the experimental equipment is shown in Fig. 4.1.

72

CO2

H2

He

Reactor

Furnace

De-

humidifier

NDIR (CO)

Analyzer

Catalyst

Fritted

Quartz

Thermocouple

Mass

Spec.

Figure 4.1: Schematic of experimental setup used for catalytic testing of the RWGS reaction

The yield of CO was calculated using the following formula:

Yield of CO (%) = [CO]out / [CO2]in x 100% (4.4)

The quantitative measurements obtained using the NDIR CO analyzer were used to obtain the

concentration of CO coming out of the reactor ([CO]out). The mass spectrometer identified any

by-products that were formed via side reactions such as CO methanation. The mass spectrometer

indicated the presence of gases with a molecular weight of up to 50 atomic units and had a

detection limit of 50 ppm.

The reactant partial pressures were also varied during experiments using the SDC

supported Cu50Ni50 catalyst. First, the H2 partial pressure was kept constant at 1 kPa while the

CO2 partial pressure was varied from 1 to 4 times the H2 partial pressure. Then, the CO2 partial

pressure was kept constant at 1 kPa while the H2 partial pressure varied from 1 to 4 times the CO

73

partial pressure. The total flow rate of the gaseous stream was kept constant at 510 mL·min-1

, by

adjusting the He flow rate. The total gas pressure remained constant at 1 atm in all experiments.

Thermal stability experiments were performed on the Cu50Ni50/SDC catalyst by

maintaining the temperature at 700°C for 24 hrs at a total pressure of 1 atm and a total flow rate

of 510 mL·min-1

. The gas composition was 1 kPa H2, 1 kPa CO2 with the balance being He.

Subsequently, the temperature was decreased to 600°C and maintained constant for another 24

hrs, for a total catalyst time-on-stream of 48 hrs.

Thermodynamic equilibrium calculations were performed using UniSim simulation

software. A Gibbs reactor calculation was used to determine the equilibrium conversion at the

operating conditions. Equilibrium conversions were compared with experimental results.


4.3.1 Physical Characterization of Cu50Ni50

X-ray diffraction spectra of Cu50Ni50 nanoparticles are shown in Figure 4.2. Ni and Cu

were the major species and no nitrate salts were present. X-ray diffraction peaks having 2θ

values of 43.39° and 44.41° were attributed respectively to reflections from the 111 plane of a

Cu rich alloy and from the 111 plane of a Ni rich alloy as shown in [24]. The crystalline size was

obtained using Scherrer’s formula and gave 20.5 nm for the Cu rich (111) peak and 9.6 nm for

the Ni rich (111) peak.

74

Figure 4.2: XRD of Cu50Ni50 colloidal solution

SEM images were used to measure particle size, shape and dispersion on a carbon

support. A carbon support was used for SEM because carbon would have less electron static

charging than YSZ or SDC. Two images are shown in Fig. 4.3 for both COMPO, composition,

and LEI, lower secondary electron image, settings.

Figure 4.3: SEM of Cu50Ni50/C in a) LEI mode and b) COMPO mode

Light coloured regions are indicative of Cu50Ni50 particles and the darker coloured

background represents the C support. These images show good dispersion of the metal on the

support and also indicate agglomeration. Anti-agglomerating agents like polyvinylpyrrolidone

(PVP) were not used to minimize the cost of the synthesis materials. A typical particle size

observed in multiple SEM spectra was 30 nm.

38 40 42 44 46 48 50 52 54

2

XR

D I

nte

nsity [

a.u

.]

a))

b))

75

4.3.2 Catalytic Performance

The only observable components in the gas stream entering the mass spectrometer were

CO2, H2, CO, trace amounts of H2O and the carrier gas, He. These results indicate that CO was

the main product having a typical concentration of 2000 ppm. Other products including CH4 had

concentrations of less than the detection limit of the spectrometer, 50 ppm. These results with

the CuNi/YSZ and CuNi/SDC catalysts were completely consistent with previous work using

CuNi/C and CuNi/Al2O3 catalysts [24].

CO and CH4 were the expected products with pure copper catalysts [9], [11], [14], [26]

and pure nickel catalysts [14] respectively. The absence of CH4 in this work might have been

caused by the enhanced copper concentration on the catalyst surface. XPS analysis reported

previously for Cu50Ni50/C catalysts [24] indicated that the surface atomic Cu/Ni was “2” , even

though the bulk atomic Cu/Ni rate was “1”. Perhaps any CO formed on Ni atom sites may have

“spilled over” to Cu atom sites and desorbed prior to further hydrogenation to CH4.


–1, 50

mg of catalyst: Pt/YSZ, 1 wt% catalyst where = 1st cycle, = 2

nd cycle and = 3

rd cycle

The data in Figure 4.4 were obtained when the RWGS reaction was performed with a catalyst

composed of platinum supported on a YSZ support. The CO yields obtained during all of the

0

5

10

15

20

25

30

35

40

45

50

55

350 400 450 500 550 600 650 700 750

Yie

ld o

f C

O (

%)

Temperature (˚C)

76

cycles were similar at 400 and 450C. In contrast the CO yields at 650 and 700C for the third

cycle were slightly less than those during the first and second cycles.


, 50

mg of catalyst: Cu50Ni50/YSZ, 10 wt% catalyst where = 1st cycle, = 2

nd cycle and = 3

rd

cycle

Figure 4.5 demonstrates the results obtained when the RWGS reaction was performed

over a catalyst composed of CuNi nanoparticles supported on a YSZ support. The CO yields

obtained during the second and third cycles were similar and were somewhat less than those

obtained during the first cycle. It indicates that while some deactivation had occurred, the

catalysts became thermally stable after the first cycle. Cu alone is known to be unstable at high

temperatures [9], [11], [26]. It may be that the Ni in Cu50Ni50 stabilizes Cu so that the Ni

provides the desired thermal stability and the Cu provides the desired CO yields.

0

5

10

15

20

25

30

35

40

45

50

55

350 400 450 500 550 600 650 700 750

Yie

ld o

f C

O (

%)

Temperature (˚C)

77


–1, 50

mg of catalyst: Pt/SDC, 1 wt% catalyst where = 1st cycle, = 2

nd cycle and = 3

rd cycle

The data in Figure 4.6 shows the results obtained when Pt supported on a samarium-

doped ceria support was used for the RWGS reaction. The CO yields obtained during the second

and third cycles were superior to those obtained during the first cycle. This might be caused by a

reduction of Pt oxide particles on the surface of the catalyst to metal Pt. Once reduced, its

catalytic performance appears to be stable. The fact that similar CO yields were obtained during

the second and third cycles indicates that no deactivation was apparent. The thermal stability

shown by Pt was expected based on previous reports [16], [27]–[29]. The CO yields obtained

with the Pt/SDC catalyst in Figure 4.6 were superior to those obtained with the Pt/YSZ catalyst

in Figure 4.4.

0

5

10

15

20

25

30

35

40

45

50

55

350 400 450 500 550 600 650 700 750

Yie

ld o

f C

O (

%)

Temperature (˚C)

78


–1, 50

mg of catalyst: Cu50Ni50/SDC, 10 wt% catalyst where = 1st cycle, = 2

nd cycle and = 3

rd

cycle

The data in Figure 4.7 were obtained when CuNi nanoparticles supported on a samarium

doped ceria support was used for the RWGS reaction. The CO yields obtained during all of the

cycles were virtually the same, indicating that the catalyst was thermally stable and that there

was no deactivation. The CO yields obtained with the Cu50Ni50 /SDC catalyst in Figure 4.7 were

superior to those obtained with the Cu50Ni50/YSZ catalyst in Figure 4.5.

Figure 4.8: Average yield of CO for the RWGS reaction using Pt supported on SDC and YSZ at

1 atm. Total flow rate of 510mL∙min-1

, PCO2 = PH2 = 1kPa, balance He. Where = equilibrium,

= Pt/SDC and = Pt/YSZ

0

5

10

15

20

25

30

35

40

45

50

55

350 400 450 500 550 600 650 700 750

Yie

ld o

f C

O (

%)

Temperature (˚C)

0

5

10

15

20

25

30

35

40

45

50

55

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

79

The average yields of the two Pt catalysts are compared with the equilibrium CO yield in

Figure 8. The CO yield with the Pt/SDC catalyst is better than that with the Pt/YSZ catalyst at

all temperatures. The CO yield with the Pt/YSZ catalyst was always less than the equilibrium

CO yield. Although the CO yield with the Pt/SDC catalyst was less than the equilibrium yield at

lower temperatures, at 700C the equilibrium CO yield was attained using the Pt/SDC catalyst.

Figure 4.9: Average yield of CO for the RWGS reaction using Cu50Ni50 supported on SDC and

YSZ at 1 atm. Total flow rate of 510 mL∙min-1

, PCO2 = PH2 = 1kPa, balance He. Where =

equilibrium, = Cu50Ni50 / SDC and = Cu50Ni50 / YSZ

The average yields of the two Cu50Ni50 catalysts are compared with the equilibrium yield

in Figures 4.9. The CO yield with the Cu50Ni50/SDC catalyst is better than that with the

Cu50Ni50/YSZ catalyst at all temperatures. In addition the CO yields with both catalysts are less

than the equilibrium CO yields at all temperatures.

For both Pt and Cu50Ni50 catalysts the CO yields were greater with the SDC support than

with the YSZ support. This is consistent with data reported by Ismail [19] for SDC and YSZ

supports that did not contain Cu50Ni50. [15]. The CO yields may be related to the concentration

of bulk phase oxygen vacancies in the two supports, 0.1 for SDC and 0.074 for YSZ.

0

5

10

15

20

25

30

35

40

45

50

55

350 400 450 500 550 600 650 700

Yie

ld o

f C

O (

%)

Temperature (˚C)

80

Two supports having no bulk phase oxygen vacancies, C and γ-Al2O3, were investigated

in previous work [24]. Their results with Cu50Ni50 nanoparticles were combined with the

Cu50Ni50 nanoparticle results on YSZ and SDC supports and are shown in Figure 4.10. The CO

yields in Figure 4.10 are definitely correlated with the oxygen vacancy content of the support.

Other reaction results have also been correlated with oxygen vacancies. For example, the results

Pekridis et al. [16] obtained using a solid oxide fuel cell (SOFC) at 650-800°C were explained in

terms of oxygen vacancies.

Figure 4.10: CO Yield versus bulk phase oxygen content in supports containing Cu50Ni50



He

The catalysts containing carbon and -Al2O3 supports had measurable CO yields even

though they did not contain bulk phase oxygen vacancies. The surfaces of -Al2O3 contain

oxygen vacancies. During the preparation of -Al2O3, hydroxyl groups are formed on the solid

surface. When the solid is heated some of the hydroxyl groups combine to form a water

molecule that enters the gas phase [30]. An oxygen anion and an oxygen vacancy are left behind

on the solid alumina surface. The surfaces of carbon particles contain various oxygen groups

including hydroxyls and carboxyls. When hydrogen is present at the reaction conditions of these

experiments, water can be formed and enter the gas phase, leaving surface oxygen vacancies on

10

15

20

25

30

35

40

45

0 0.05 0.1

Yie

ld o

f C

O (

%)

Bulk Phase Oxygen Vacancy Content

81

the surface. Although the carbon and -Al2O3 supports do not contain bulk phase oxygen

vacancies, they do contain surface oxygen vacancies. The presence of surface oxygen vacancies

would be consistent with the CO yields they obtained.

A diagram illustrating the bi-functional nature of the catalyst is shown in Figure 4.11. A

catalytic mechanism can be suggested that involves the adsorption of hydrogen on the CuNi

metal phases and the adsorption of carbon dioxide on the support. The amount of H2 adsorbed

on pure Ni is known to be five times greater than that adsorbed on pure Cu [31]. The amount of

hydrogen adsorbed on CuNi alloys does not vary much with alloy composition and is

approximately one-third of that adsorbed on pure Ni [31]. CO2 adsorption on YSZ has been

reported as a bicarbonate species [32]. Although we are not aware of any SDC data, CO2

adsorption has also been reported on gallium doped ceria as a bicarbonate species [33].

Based on the above, the adsorption of the reactants can be expressed in the form of

equations. CO2 adsorbed on support oxygen vacancies, Vox(S), might be represented as shown in

CO2(g) + Vox(S) = CO2(ads)-Sup (4.5)

Equation 4.5. Hydrogen from the gas phase might be dissociatively adsorbed on the surface of

the metal, where the electron on a dissociated hydrogen radical is shared with an empty orbital in

the conduction band of the metal, Me, as shown in Equation 4.6.

H2(g) + 2 Me = 2 H(ads)-Me (4.6)

Eley-Rideal mechanisms might be operative in two ways. Hydrogen from the gas phase might

react with adsorbed CO2 as shown in Equations 4.7 and 4.8.

½ H2(g) + CO2(ads)-Sup = OCOH(ads)-Sup (4.7)

½ H2(g) + OCOH(ads)-Sup = CO(g) + H2(g) + Vox(S) (4.8)

82

Since the CO2 adsorbed on the support reacts with hydrogen in the gas phase, it could reduce

CO2 to CO even if the catalyst did not contain metal particles, as was shown in other work

reported by Ismail [15]. In his work, he described both YSZ and SDC catalysts that converted

CO2 to other products without a metallic component in the catalyst.

A second Eley-Rideal mechanism would occur if CO2 in the gas phase reacted with

adsorbed H2 as shown in Equation 4.9. Vesselli et al [34], [35] has described the occurrence of

the Eley-Rideal mechanism in RWGS reactions.

CO2(g) + 2 H(ads)-Me = CO(g) + H2O(g) + 2 Me (4.9)

A Langmuir-Hinshelwood mechanism might also be possible. The adsorbed hydrogen

might react with the adsorbed CO2 at the three-phase (metal-support-gas) boundary, as shown in

Equations 4.10. Alternatively, the hydrogen adsorbed on the metal surface diffuses from the

metal surface unto the support surface prior to reacting via Equation 4.10 (hydrogen spillover).

The resulting species, OCOH(ads), might diffuse over a support surface by hopping from one

oxygen vacancy to another until two OCOH(ads) species were adjacent to one another which

H(ads)-Me + CO2(ads)-Sup = OCOH(ads)-Sup + Me (4.10)

might yield the reaction via Equation 4.11.

2 OCOH(ads)-Sup = CO2(ads)-Sup + Vox(S) + CO(g) + H2O(g) (4.11)

The rate of surface diffusion might be influenced by the concentration of oxygen vacancies on

the support surface. In this case the reaction would occur on an extended region of the support

surface surrounding the three-phase boundary. Pekridis et al. [5] proposed a similar Langmuir-

Hinshelwood mechanism to explain results obtained with a solid oxide fuel cell.

83


operating through an Eley-Rideal and a Langmuir Hinshelwood mechanism

4.3.3. Partial Pressure Variation and Stability Measurements

The results of experiments in which the partial pressure of hydrogen was varied at

constant CO2 partial pressure are shown in Figure 4.12. Four different temperatures were used.

At each temperature there was an increase in CO yield as the hydrogen partial pressure

increased. The equilibrium constant, KRWGS, shown in Equation 4.12, will be a different constant

at each temperature.

KRWGS = [CO][H2O] / [CO2] [H2] (4.12)

Both the equilibrium CO partial pressure and the equilibrium CO yield will increase as the

hydrogen partial pressure increases. Since the hydrogen partial pressure in the experiments

increased by a factor four, the equilibrium CO yield would also be expected to increase by a

factor of four, if the reaction was equilibrium limited. Since the increase in CO yield with H2

partial pressure in Figure 12 is much less than a factor of four at all temperatures it can be

suggested that the increase in CO yield may not entirely be caused by changes in equilibrium

conditions.

Another possibility is that the reaction is kinetically limited. For a gas phase reaction,

such as the Eley-Rideal mechanism, the rate might be expressed as shown in Equation 4.13. The

Rate = k * ( pH2 )N (4.13)

CO2 CO2

CO2(g) H2(g)

CO(g) + H2O(g)

84

data in Figure 4.13 are consistent with the notion that the increase in hydrogen partial pressure

caused an increase in the kinetics of the reaction, because the CO yield increased as the hydrogen

partial pressure increased.

CH4 production was observed at H2:CO2 ratios greater than 2. CH4 is undesirable for two

reasons. The selling price of CH4 is generally less than the cost of supplying the H2 from which

it is made. Furthermore, any increase in the amount of carbon used to make CH4 means there is

less carbon used to make CO that can be used in the Fischer-Tropsch process to make valuable

hydrocarbons.


960800 h–1

. PCO2 = cst. Total flow rate of 510 mL∙min-1

. Where = 700°C, = 600°C, =

500°C and = 400°C

The results of experiments in which the partial pressure of carbon dioxide was varied at

constant H2 partial pressure are shown in Figure 4.13. Four different temperatures were used.

At each temperature there was a decrease in CO yield as the CO2 partial pressure increased. If

the concentration of adsorbed CO2 on the support surface increased with increasing CO2 partial

pressure, the surface concentration of oxygen vacancies would decrease. Fewer oxygen

vacancies on the support surface would suggest that the rate of diffusion of OCOH(ads) species

0

10

20

30

40

50

60

70

0 2 4

Yie

ld o

f C

O (

%)

H2 Partial Pressure

85

might decrease. Therefore the decrease in CO yield observed in Figure 4.13 is consistent with

the hypothesis that the concentration of oxygen vacancies on the support surface influences the

rate of surface diffusion of OCOH(ads) reaction intermediate species.


960800 h–1

. PH2 = cst. Total flow rate of 510 mL∙min-1

. Where = 700°C, = 600°C, = 500°C

and = 400°C

Time-on-stream experiments were performed in order to examine the catalyst’s thermal

stability over a 48-hour period, 24 hours at 700°C and subsequently another 24 hours at 600°C.

The results are shown in Figure 4.14. No deactivation was apparent.

This is a significant achievement in the use of a Cu catalyst for the RWGS reaction.

Previous studies suggested that Cu is inadequate for use at high temperatures because of its

tendency to deactivate over time [11], [12]. However, the results in Figure 4.14 demonstrated CO

yields that approached the equilibrium CO yield and the results in Figure 4.14 suggested long-

term thermal stability.

0

10

20

30

40

50

60

70

0 2 4

Yie

ld o

f C

O (

%)

CO2 Partial Pressure

86


700°C and b) 600°C. GHSV = 960800 h–1


The favorable results reported here can probably be attributed to Cu being used as a CuNi

alloy catalyst. The Ni component is most likely responsible for the thermal stability. The 100 %

selectivity to CO may be attributed to the CuNi alloy surface having a much greater

concentration of Cu than the bulk CuNi alloy. A large surface concentration of Cu diminishes

the tendency for CH4 to be formed on Ni.

All of the results reported here were obtained at small partial pressures of hydrogen. The

experiments performed at H2 partial pressures of 2 kPa and greater showed that CH4 was formed

and that 100 % selectivity to CO was not obtained. There may be merit in performing future

experiments at greater hydrogen partial pressures with CuNi alloy catalysts that have larger

Cu/Ni ratios than the one used in this investigation, since it is known [24] that the surface Cu/Ni

ratio increases non-linearly with the bulk Cu/Ni ratio.

4.4 Summary and Conclusion

Cu50Ni50 nanoparticles were successfully synthesized using a modified polyol method

that caused both the copper and nickel compounds to be reduced simultaneously and thereby

0

5

10

15

20

25

30

35

40

45

50

55

0 5 10 15 20 25 30 35 40 45

Yie

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f C

O (

%)

Time Elapsed (Hours)

a) b)

87

form CuNi alloy nanoparticles. CO yields with Pt/SDC catalysts equaled equilibrium CO yields

at 650 and 700C. In contrast CO yields with Cu50Ni50/SDC catalysts were about 90% of the

equilibrium CO yields at 650 and 700C. Catalysts with YSZ supports had CO yields that were

approximately 55% of the CO yields obtained with catalysts having SDC supports.

The catalyst selectivity to CO was essentially 100% for the experiments performed at a

hydrogen partial pressure of 1 kPa. That may have been caused in part by the Cu content of the

catalyst surface being substantially greater than that in the bulk catalyst. When experiments

were performed using hydrogen partial pressures of 2 kPa or greater, an increase in the CO yield

was observed but methane was observed in the product gas, thereby decreasing the selectivity to

CO. When the partial pressure of CO2 was increased the CO yield decreased. The reaction

results were explained using a combination of Eley-Rideal and Langmuir-Hinshelwood

mechanisms that involved adsorption on the metal surface and the concentration of oxygen

vacancies in the support. Finally the Cu50Ni50/SDC catalyst was found to be thermally stable for

48 hours at 650/700C.

Because the costs of Cu and Ni are substantially less than for Pt and because the

performance of the Cu50Ni50/SDC catalysts approached the performance of Pt/SDC catalysts

more extensive testing of supported CuNi catalysts is warranted.

4.5 Acknowledgements

Both the Natural Sciences and Engineering Research Council (NSERC) and Phoenix

Canada Oil Company Limited are gratefully acknowledged for their financial support.

88

4.6 References

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[10] C. S. Chen, J. H. Wu, and T. W. Lai, “Carbon Dioxide Hydrogenation on Cu

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322–332, 1994.

[20] W. Dow, Y. Wang, and T.-J. Huang, “Yttria-Stabilized Zirconia Supported Copper Oxide

Catalyst,” J. Catal., vol. 170, no. 0135, pp. 155–170, 1996.

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[22] H. Yahiro, K. Eguchi, and H. Arai, “Electrical properties and reducibilities of ceria-rare

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performance of CuO–CeO2 catalysts in methanol steam reforming,” Appl. Catal. B

Environ., vol. 69, no. 3–4, pp. 226–234, Jan. 2007.

[27] A. Dauscher, L. Hilaire, F. Le Normand, W. Miiller, G. Maire, and A. Vasqwz,

“Characterization by XPS and XAS of Supported Pt/TiO2-CeO2 Catalysts,” vol. 16, pp.

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[28] C. M. Kalamaras, P. Panagiotopoulou, D. I. Kondarides, and A. M. Efstathiou, “Kinetic

and mechanistic studies of the water–gas shift reaction on Pt/TiO2 catalyst,” J. Catal., vol.

264, no. 2, pp. 117–129, Jun. 2009.

[29] A. Goguet, F. C. Meunier, D. Tibiletti, J. P. Breen, and R. Burch, “Spectrokinetic

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[32] E. Kock, M. Kogler, T. Bielz, B. Klo, and S. Penner, “In Situ FT-IR spectroscopic study

of CO2 and CO adsorption on Y2O3, ZrO2, and yttria-stabilized ZrO2,” J. Phys. Chem.,

2013.

[33] G. Finos, S. Collins, G. Blanco, E. del Rio, J. M. Cíes, S. Bernal, and A. Bonivardi,

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91

Chapter 5 – General Discussion

5.1 Introduction

The main goal of the Thesis was to identify a useful catalyst capable of being used

industrially. This catalyst therefore needs to be relatively inexpensive to synthesize. The catalyst

also needs to be selective to CO production, have high CO yields and be stable at high operating

temperatures.

CuNi nanoparticles were chosen as a potential catalyst for the RWGS reaction. The main

goal was to combine high CO selectivity achieved when using Cu nanoparticles [1], [2] and good

stability at high temperatures when using Ni nanoparticles [3]–[5]. An alloy of the two

compounds was considered to be a possible stable and selective catalyst capable of achieving

high CO yields.

Finding a simple and efficient synthesis method for producing CuNi nanoparticles was

one of the objectives of Chapter 3. These nanoparticles were then characterised using X-ray

diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscope

(EDS) and x-ray photoelectron spectroscopy (XPS) with the goal of identifying the size, content

and morphology of the nanoparticles.

One batch of nanoparticles was synthesized and repeatedly used throughout the research.

Because of this, the characterizations shown throughout Chapter 3 are applicable when analyzing

results in Chapter 4. Using the same CuNi nanoparticles was necessary to study the effect of

different supports on the RWGS reaction. In Chapter 3, the metal was supported on powders

containing no stoichiometric oxygen vacancies (carbon and gamma-alumina). In Chapter 4, it

92

was deposited on supports containing stoichiometric oxygen vacancies (samarium-doped ceria,

SDC, and yttria-stabilized zirconia, YSZ).

Another objective was to compare the performance of the CuNi nanocatalyst to a Pt

nanocatalyst when both are deposited on the same supports. Pt was used as basis of comparison

because Pt group metals (PGM) are usually good hydrogenation catalysts [2]. The Pt

nanoparticles were already synthesized and characterised previously and had been shown to be a

good working catalysts for other reactions [6]–[8].

Different CuxNi1-x (x=20, 50, 80) ratios were used in Chapter 3. The best working Cu-Ni

ratio was chosen for further testing which was described in Chapter 4 on supports containing

stoichiometric oxygen vacancies.


The physical characterization of the CuNi nanoparticles produced several important

results. XPS analysis done in Chapter 3 showed enriched Cu on the surface of the three CuxNi1-x

(x=20,50,80) metals. The XRD spectra showed shifts in both Cu (111) and Ni (111) peaks for all

three CuNi ratios which indicated the presence of alloyed species.

Results obtained in Chapter 3 showed signs of stability at high temperatures. The

Cu50Ni50/C catalyst was cycled 3 times for a total on-stream time of 24h showing no signs of

deactivation after the second cycle. High selectivity towards CO was also observed since no CH4

was measured during any of the experiments during Chapter 3.

The Cu50Ni50 metal catalyst was chosen in Chapter 4 due to its good yield and high

stability. This catalyst was deposited on SDC and YSZ. Similar to the results obtained in Chapter

3, the CuNi catalysts showed no deactivation after the second cycle. In addition, the catalyst was

93

tested for a total of 48 h at 600 and 700°C showing no deactivation. This can be seen in Figure

5.1 below. CH4 was not detected during any of the experiments when H2 and CO2 were fed

stoichiometrically (PH2/PCO2 = 1).


700°C and b) 600°C. GHSV = 960800 h–1


These results are explained using the physical characterization obtained in Chapter 3. It

is hypothesized that CO selectivity is caused by the enrichment of Cu that was measured on the

surface of the catalyst. In addition, it is hypothesized that the high stability of the catalyst may be

caused by the presence of an alloyed metal. This was also reported in other research when Fe

was used as stabilizer [9].

Support interaction has been shown to be an important aspect of catalysis. For example

yield can be dependent on the support. The influence of catalyst-support interaction can be seen

when comparing the data of Chapter 3 to the data in Chapter 4. Supports containing no

stoichiometric oxygen vacancies were used in Chapter 3 (C and γ-Al2O3) while supports

containing stoichiometric oxygen vacancies were used in Chapter 4 (YSZ and SDC). The effect

of oxygen vacancies on CO yield from the RWGS reaction is evident in Figure 5.2. This is

0

5

10

15

20

25

30

35

40

45

50

55

0 5 10 15 20 25 30 35 40 45

Yie

ld o

f C

O (

%)

Time Elapsed (Hours)

a) b)

94

consistent with other research reports which showed that the WGS reaction was accelerated

using ZrO2 containing oxygen vacancies [10]. Pt was also tested with the different supports.

Similarly to CuNi, Pt showed increasing yield with supports containing oxygen vacancies as

shown in Figure 5.2.

Figure 5.2: CO yield versus bulk phase oxygen content in supports containing Cu50Ni50



He. Dashed line = equilibrium

A reaction mechanism was suggested in Chapter 4 which could explain the increased

yield of CO obtained when using supports containing oxygen vacancies. Both Langmuir-

Hinshelwood and Eley-Rideal mechanisms would be occurring in the reaction. A diagram shown

in Figure 5.3 demonstrates how both pathways may be possible. It was hypothesized that CO2

could adsorb on the surface of the support through its oxygen vacancy and subsequently could

react with either gaseous H2 or adsorbed H.

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Bulk Phase Oxygen Vacancy Content

95


operating through an Eley-Rideal and a Langmuir Hinshelwood mechanism

Both Chapters 3 and 4 compare results obtained using CuNi nanoparticle catalysts with

Pt nanoparticle catalysts. In both cases, the yield of CO is comparable. Figure 5.4 compares

CuNi to Pt when SDC is used as a support. Furthermore, it is speculated that CO yields might be

increased to the equilibrium yield achieved with Pt that was obtained at 700°. Improving the

synthesis method through the use of polyvinilpyrrolidone (PVP) or through pH and temperature

variations might cause modifications to the physical properties of the nanoparticles making them

different shapes or sizes [11]. These variations have been reported to be useful according to

reports in literature [12]. Also, changing the metal loading of the catalyst may cause improved

performance.

CO2 CO2

CO2(g) H2(g)

CO(g) + H2O(g)

96

Figure 5.4: Average yield of CO for the RWGS reaction using Cu50Ni50 and Pt supported on

SDC at 1 atm. Total flow rate of 510 mL∙min-1

, PCO2 = PH2 = 1kPa, balance He. Where =

equilibrium, = Pt / SDC and = Cu50Ni50 / SDC

5.3 Conclusion

By combining the information in Chapters 3 and 4, several trends in the results were

illustrated. They included high selectivity, stability and increased performance when using

supports containing oxygen vacancies. A diagram was prepared to illustrate some possible

reaction mechanisms that might occur in the RWGS reaction. In the future, the performance of a

modified CuNi metal catalyst for the RWGS reaction might approach that of the Pt catalyst with

all four supports (C, γ-Al2O3, YSZ and SDC). Finally, an important advantage of CuNi alloys is

that they cost substantially less than Pt

5.4 References







0

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30

35

40

45

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55

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Yie

ld o

f C

O (

%)

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97

[4] J. M. Bermúdez, B. Fidalgo, a. Arenillas, and J. a. Menéndez, “CO2 reforming of coke

oven gas over a Ni/γAl2O3 catalyst to produce syngas for methanol synthesis,” Fuel, vol.

94, pp. 197–203, Apr. 2012.

[5] C. Schild, A. Wokaun, R. A. Koeppel, and A. Baiker, “CO2 Hydrogenation over Nickel /

Zirconia Catalysts from Amorphous Precursors : On the Mechanism of Methane

Formation,” J. Phys. Chem., vol. 95, pp. 6341–6346, 1991.




E14, 2012.




pp. 996–1002, Jul. 2013.




43–57, 2011.



106, Jan. 2004.




[11] K. J. Carroll, J. U. Reveles, M. D. Shultz, S. N. Khanna, and E. E. Carpenter, “Preparation

of Elemental Cu and Ni Nanoparticles by the Polyol Method: An Experimental and

Theoretical Approach,” J. Phys. Chem. C, vol. 115, no. 6, pp. 2656–2664, Feb. 2011.



vol. 464–465, pp. 87–94, Aug. 2013.

98

Chapter 6: Conclusion

6.1 Summary of Results

The objectives of the Thesis were separated into four sections and each addressed

throughout this work. The following section aims to summarize and conclude the findings

achieved by completing the prescribed objectives. The objectives were as follows:

Objective 1: Develop a synthesis method producing CuxNi1-x nanoparticles

Objective 2: Characterize the CuxNi1-x catalysts

Objective 3: Establish a “best case scenario” using an established noble metal (Pt) based

nanocatalyst

Objective 4: Investigate the performance of CuxNi1-x nanoparticles deposited on supports

having varying oxygen vacancy content for the RWGS reaction

6.1.1 Objective 1: Synthesis of CuxNi1-x nanoparticles

CuNi nanoparticles were successfully synthesized using a novel polyol method. This

method was simple, time efficient and yielded nanoparticles of a certain size distribution (4 nm -

100 nm) without the use of stabilizers such as PVP (polyvinylpyrrolidone). Cu and Ni nitrates

were used as salt precursors dissolved in ethylene glycol. NaOH was used as pH adjuster

increasing both solutions' pH to 11. The nickel nitrate solution was first refluxed at 196C and

showed no signs of reduction to metallic nickel. The copper solution (room temperature) was

then added resulting in the reduction of both species to metallic nanoparticles. Once cooled, the

colloids were stored in their ethylene glycol solution.

The particles were deposited using an impregnation technique on a total of 4 supports:

samarium-doped ceria, SDC, yttria-stabilized zirconia, YSZ, gamma-alumina, γ-Al2O3, and

99

carbon, C. The technique involved inserting a certain amount of the support in a beaker and

injecting the colloids on top of the solution. The mixture was then sonicated and mixed for 24

hours. De-ionized water was then used to wash the powder removing impurities. The mixture

was washed and centrifuged 10 times.

6.1.2 Objective 2: Characterization of the CuxNi1-x catalysts

Characterization of the CuNi nanoparticles was done using several techniques including:

scanning electron microscopy, SEM, energy-dispersive X-ray spectroscope, EDS, x-ray

diffraction, XRD, and x-ray photoelectectron spectroscopy, XPS. Several findings were obtained

using the aforementioned characterization techniques.

SEM micrographs of the carbon supported CuNi nanoparticles were taken in order to

examine particle morphology and distribution. Spherical particles were the main shape observed

in all micrographs. Typical SEM particle sizes observed are shown in Table. 6.1. Some

agglomeration was observed principally due to the lack of stabilizer used during the synthesis.

There were no observable differences between the particles before exposure to reaction

conditions and after.

Table 6.1: CuxNi1-x / Carbon physical characteristics Catalyst XRD Crystalline

size (nm)

SEM Typical

particle size (nm)

Cu80Ni20 / C 30.2 64.4

Cu50Ni50 / C 24 53.4

Cu20Ni80 / C 16.7 41.1

EDS was used to obtain the bulk composition of a small sample (2-5 nanoparticles) of the

CuNi nanoparticles. There was a good agreement between the calculated ratios and those

obtained through EDS measurements.

100

XRD spectra of the CuNi colloids were taken and analyzed with the goal of confirming

the presence of both Cu and Ni metals as well as determine if the formation of an alloy was

observed. Two distinct 111 peaks were obtained for the Cu50Ni50 and Cu20Ni80 particles and only

one was observed for the Cu80Ni20 sample. All peaks were shifted inwards from pure Cu peak

(43.2 ICSD Collection Code: 53246) and the pure Ni peak (44.6 ICSD Collection Code: 43397)

suggesting that the particles contained a Cu rich solid solution and, for Cu50Ni50 and Cu20Ni80, a

Ni rich solid solution in addition to the Cu rich solid solution. No salt was observed in any of the

XRD spectra. Crystalline sizes were obtained using Scherer’s' formula and are given in Table.

6.1 for all of the CuNi colloids.

An XPS analysis of the surface composition of each of the carbon supported CuNi

catalysts was performed in order to identify to main specie found on the catalytic surface. All

three catalysts were determined to have increased Cu concentrations on the surfaces than what

was found in the bulk. This was consistent with literature [1]–[3].

6.1.3 Objective 3: Supported Pt Nanoparticles for the RWGS Reaction

Pt supported nanoparticles were previously synthesized using a synthesis method

described elsewhere [4]. The supports were C, γ-Al2O3, YSZ and SDC. 50 mg of each catalyst

was tested for the RWGS reaction under atmospheric pressure and with temperatures ranging

from 400°C to 700°C. This was repeated for 3 cycles.

Gamma-alumina proved to be unstable at high temperatures. This was consistent with

literature [5]. Otherwise, the catalysts showed signs of stability with all other supports during the

3 cycles for the temperature range used.

101

The C supported catalyst showed the lowest conversion of all supports used. There was a

clear distinction between the traditional supports (C and γ-Al2O3) and the two other supports

containing stoichiometric oxygen vacancies (YSZ and SDC). Pt/YSZ produced a good yield of

CO at all temperatures but did not approach equilibrium. Pt/SDC reached the equilibrium yield

to CO at 700°C (43.9%). The results are shown in Figure 6.1. There was no sign of CH4

production during any of the experiments.

Figure 6.1: Average yield of CO for the RWGS reaction using Pt supported on C, γ-Al2O3, SDC

and YSZ at 1 atm, PH2 = PCO2 = 1kPa, balance He with 50 mg of catalyst. Total flow rate of

mL∙min-1. Where = Equilibrium, = Pt/SDC, = Pt/YSZ, = Pt/C, = Pt/ γ-Al2O3

Oxygen vacancies found on the surface of the two supports are suggested to be the main

cause for the increased yields observed. A mechanism was also suggested in Chapter 4 which

explains the interaction of the oxygen vacancies in the RWGS reaction.

6.1.4 Objective 4: CuxNi1-x Nanoparticles Deposited on Supports Having Varying Oxygen

Vacancy Contents for the RWGS Reaction

Similar to the work conducted with the supported Pt nanoparticles, CuNi nanoparticles

were deposited on C, γ-Al2O3, YSZ and SDC. Three CuxNi1-x catalysts were used: x = 20, 50, 80.

50 mg of each catalyst was tested for the RWGS reaction at atmospheric pressure and

temperatures ranging from 400°C to 700°C each tested for 3 cycles.

0

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102

Stability was one of the key components tested in these experiments. Once again, the γ-

Al2O3 support showed a lack of stability for all metal catalysts. Cu80Ni20 and Cu20Ni80 also

showed signs of instability when deposited on C. The only carbon supported CuNi catalyst to

show consistent stability was the Cu50Ni50 catalyst. When YSZ and SDC were used as supports,

both Cu50Ni50 and Cu20Ni80 showed good stability. In addition cyclic experiments were

performed. Cu50Ni50/SDC was tested for 48 consecutive hours at 600°C and 700°C. There were

no signs of deactivation during this period.

High CO yield was another important aspect for this research. There was no observable

trend differentiating the CuNi ratios with most supports. There was however a clear distinction

between each support. Higher stoichiometric oxygen vacancy content showed increased yields

for all catalysts. This is shown in Figure 6.2. Cu50Ni50/SDC showed the highest yield to CO

among CuNi catalyst achieving 39.8% at 700°C. This was less than 4% from equilibrium which

was achieved by Pt/SDC.

Figure 6.2: Effect of oxygen vacancy on yield of CO for the RWGS reaction using Cu50Ni50

metal at 1 atm, PH2 = PCO2 = 1kPa, balance He, 50 mg of catalyst at 700°C. Total flow rate of

mL∙min-1. Dotted line = equilibrium yield

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f C

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Bulk phase Oxygen Vacancy Content

103

Methane production was closely monitored during all experiments. CH4 was expected

when using a Ni containing catalyst since Ni has the tendency to hydrogenate CO to CH4. There

was no CH4 observed during any of the experiments conducted. This was explained by the

reduced Ni concentration on the surface of all CuNi catalysts. As shown by the XPS analysis, Cu

is the dominant specie in all cases. Methane was observed when the partial pressure of H2 was

increased to 2 times that of CO2. This is consistent with what is expected stoichiometrically and

what is found in the literature [6]. Higher yields were also observed with higher H2 partial

pressures.

The Cu50Ni50/SDC catalyst was closest to achieving the equilibrium yield that was

obtained with Pt/SDC. It was found that this catalyst operated very well for extended periods of

time at high temperatures without generating CH4 as a by-product. Some modifications

suggested in section 6.2 describe how the Cu50Ni50/SDC catalyst could one day outperform the

much more expensive Pt/SDC catalyst.

6.2 Recommendations

The catalysts synthesized via a novel synthesis method described in Chapter 3 achieved

good yield, selectivity and in most cases, good stability. Nevertheless, improvements can be

made to increase the yield of CO at lower temperatures and perhaps achieve equilibrium yield

under certain conditions.

The suggested synthesis method is flexible and can be varied. NaOH concentrations can

be varied in order to examine its effect on particle size and distribution. Smaller particles have

already shown better conversion for other reactions [7]. Other additives could also be used in an

104

attempt to narrow the size distribution. For example, polyvinylpyrrolidone (PVP) is a polymer

capable of preventing agglomeration and reducing particle size during synthesis.

Gas hourly space velocity (GHSV) varied from one support to the next depending on

bulk density. Otherwise, the gas flowrate was not varied during the experiments. Lower GHSV

values can increase yields. Investigating different reactant flow rates and/or concentrations is a

method of achieving an ideal GHSV value for each catalyst. In addition, a relatively small

amount of catalyst was used (50 mg). This can also be increased.

Reaction conditions were varied. Elements such as partial pressure and temperature were

thoroughly examined. Reactor pressure remained constant. Varying operating pressures could be

done in order to increase the yield of the gaseous reactions.

The research conducted showed increased yields when using supports containing

stoichiometric oxygen vacancies. More supports containing varying oxygen vacancy contents

could be tested in order to obtain an ideal amount for the reaction. A combination of high surface

area and stoichiometric oxygen vacancy content could also be examined in hope of increasing

yield of CO.

The CuNi nanoparticles were consistently deposited using 10 metal weight percent on the

support. Examining the effect of varying this percentage could yield an ideal loading for the

RWGS reaction.

Lastly, an alternative to CuNi would be to examine other bi-metallic catalysts for this

reaction. Cu has shown to be a prominent catalyst throughout this research since it was capable

of being stabilized. Other metals such as Sn or Fe could also be used as comparison to Ni in the

goal of stabilizing Cu. Higher yields could be achieved with these alternatives.

105

6.3 Contributions to Knowledge

The RWGS reaction has been increasingly studied over the past two decades. Cu has

shown to be a selective catalyst capable of high yields but is not stable at high temperatures [8].

While Ni has also shown high conversion, it does not contain the same selective nature shown by

Cu. It does however remain stable at high temperatures [8]. Some [9] have researched Cu:Ni

metals with different ratios for the RWGS reaction at high temperatures when supported on γ-

Al2O3. That being said, stability measurements with the bimetallic catalyst were not done, nor

were the characterization of the catalyst showing the presence of an alloyed compound.

1. Information pertaining to a new synthesis method capable of synthesizing CuNi

nanoparticles containing two different solid solutions was given. This insight has shown a route

for synthesizing a Cu rich CuNi solid solution on the surface of the catalyst while also having a

Ni rich CuNi solid solution present. A Cu rich surface is important because of the possibility of

methanation caused by Ni [9]. The synthesis was repeated for 3 different CuxNi1-x (x = 20, 50,

80) ratios. Characterization for the CuNi nanoparticles was also provided.

2. The research also shows the absence of CH4 during all experiments when the partial

pressure of the reactants was kept at or below the stoichiometric ratio (PH2 = PCO2). This had not

been shown previously when Ni was involved in the RWGS reaction. Cu was also successfully

stabilized using Ni as stabilizer.

3. High yields comparable to that of noble metals (Pt) which achieved equilibrium

conversion at 700°C were obtained with the CuNi catalyst for the first time. More specifically,

the yield with the CuNi/SDC catalyst was only 4% less than the equilibrium yield at 700°C

(43.9%).

106

4. In the past Samarium-doped ceria (SDC) and yttria-stabilized zirconia (YSZ) have had

limited testing for the RWGS reaction [10], [11]. In fact, SDC had only been tested without the

presence of a metallic catalyst. This research showed the effectiveness of this support when it

was combined with a metal catalyst. This research demonstrated the importance of oxygen

vacancies generated when samarium is inserted in the crystal lattice of ceria.

107

6.4 References



490, 1976.







Soc., vol. 126, pp. 8028–37, Jun. 2004.

[5] R. K. Oberlander, “Aluminas for Catalysts: Their preparation and properties,” Appl.

Industiral Catal., vol. 3, p. 67, 1984.



Apr. 2013.



vol. 464–465, pp. 87–94, Aug. 2013.










vol. 127, no. 1–4, pp. 337–346, Sep. 2007.

108

Appendices:

This section compliments the discussion found in Chapters 3 and 4. All information

pertinent to the Thesis that is not contained in either Chapter 3 or 4 will be discussed here. Both

equipment and experimental methodologies were kept constant throughout all experiments and

can be found in Chapters 3 and 4.

A.1 Yttria-Stabilized Zirconia for the RWGS reaction using CuxNi1-x (x = 20, 50 and 80)

nanoparticles

Yttria-stabilized Zirconia (YSZ) was used as support containing stoichiometric oxygen

vacancies for all 3 CuxNi1-x (x = 20, 50, 80) catalysts and tested for the RWGS reaction between

400°C and 700°C. In comparison to ZrO2, YSZ has 0.074 oxygen vacancies per cation. Results

obtained in Chapter 4 demonstrated that the increase in oxygen vacancy content may contribute

to a higher yield of CO.

The results shown in Figure A.1 represent the 3rd

cycle of each catalyst. Both the Ni rich

and the Cu rich catalysts showed higher yield of CO than the equimassic ratio. This observation

is consistent with what was observed in Chapter 3 when the same metal was deposited on

Carbon. The Cu rich catalyst also showed the best yield of CO up until 700°C at which the

deactivation of the catalyst caused it to drop below that of the Ni rich ratio, as shown in Figure

A.1.

The Ni rich catalyst remains a good working catalyst despite its high Ni content. This

aspect is discussed in Chapter 3. XPS measurements demonstrated high Cu concentrations on the

surface of each catalyst. In fact, the Cu surface concentration is over 4 times what it should be

theoretically for the bulk Cu20Ni80 metal. This is shown in Table A.1. In addition, there were no

109

sign of CH4 formation during any of the experiments which would have normally been

favourable with Ni rich surfaces [1], [2] that did not contain Cu.

There were small signs of deactivation for the Cu rich catalyst (Cu80Ni20) as seen in

Figure A.2 a. Otherwise, Cu50Ni50/YSZ and Cu20Ni80/YSZ showed constant yield of CO after the

first cycle.

Figure A.1: RWGS reaction at 1atm, PH2 = PCO2 = 1 kPa, balance He, GHSV = 716000h

-1 , 50

mg of catalyst: = Cu80Ni20/YSZ, = Cu50Ni50/YSZ and = Cu20Ni80/YSZ. Dotted line is

equilibrium yield

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110

Figure A.2 : RWGS reaction at 1atm, PH2 = PCO2 = 1 kPa, balance He, GHSV = 716000 h

-1 over

a) Cu80Ni20/YSZ b) Cu50Ni50/YSZ c) Cu20Ni80/YSZ, where = 1st cycle, = 2

nd cycle and =

3rd

cycle

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Temperature (˚C)

b)

c)

111

Table A.1: Cu-Ni surface ratio measurements using XPS

YSZ showed good, stable yield of CO at high temperatures for all three CuxNi1-x

catalysts. Oxygen vacancies found within the crystal lattice of YSZ is thought to be the main

contributor to this increase in yield and is further discussed in Chapter 4. Cu rich surfaces are

also believed to cause the high CO yield as well as be the main reason why no CH4 was

observed. Ni is believed to be used as a stabilizing agent permitting the use of the Cu20Ni80/YSZ

and the Cu50Ni50/YSZ catalysts at high temperatures (700°C).

A.2 Samarium-Doped Ceria for the RWGS reaction using CuxNi1-x nanoparticles

Samarium-doped Ceria (SDC) was used as a support containing more stoichiometric

oxygen vacancies than YSZ (0.074 per cation). In comparison to CeO2, SDC has 0.1 oxygen

vacancy per cation. The data in Chapter 4 illustrated the increase in yield of CO when SDC was

used as the support.

Figure A.3 compares the 3RD

cycle of all 3 CuxNi1-x / SDC (x = 20, 50, 80) catalysts used

between 400°C and 700°C. All 3 CuxNi1-x catalysts exhibit high CO yield at 700°C. Both

Cu50Ni50/SDC and Cu20Ni80/SDC are found within 5% of the equilibrium yield. A maximum of

39.8% yield of CO is observed using the equi-massic mixture. The copper rich catalyst

(Cu80Ni20/SDC) and the Ni rich catalysts have slightly less yield of CO at the same temperature

(35.2% and 38.4% respectively).

Theoretical

Cu/Ni ratio

Actual Surface

Cu/Ni ratio

Percentage

Increase

Cu20Ni80 0.25 1.2 380%

Cu50Ni50 1 2 100%

Cu80Ni20 4 4.2 5%

112

Once again, slight deactivation was observed using the Cu rich (Cu80Ni20) metal catalyst

over three cycles. Other CuxNi1-x catalysts (Cu50Ni50 and Cu20Ni80) were stable at higher

temperatures during all 3 cycles. These results are demonstrated in Figure A.4.


, 50

mg of catalyst: = Cu80Ni20/SDC, = Cu50Ni50/SDC and = Cu20Ni80/SDC. Dotted line is

equilibrium yield

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113

Figure A.4: RWGS reaction at 1atm, PH2 = PCO2 = 1 kPa, balance He, GHSV = 960800 h

-1 over

a) Cu80Ni20/SDC b) Cu50Ni50/SDC c) Cu20Ni80/SDC, where = 1st cycle, = 2

nd cycle and =

3rd

cycle

The high yield of CO observed with high Ni content is once again attributed to the strong

presence of Cu on the surface of the catalyst. In addition, both Cu50Ni50/SDC and Cu20Ni80/SDC

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a)

114

showed good stability within the temperature range used. No CH4 was observed during any of

the experiments.

SDC showed high yields during the experiments and approached equilibrium at high

temperatures (<650°C). This is may be caused by the oxygen vacancies present when Samarium

is inserted within the Ceria crystal lattice. These oxygen vacancies may actively participate in the

reaction, as shown in Chapter 4.

A.3 Conclusion

SDC demonstrated better yields of CO at most temperatures using the three CuxNi1-x

metals. These trends were previously observed in Chapter 4 with only the equi-massic mixture.

Similar observations were found by Ismail [3]. He tested both supports without the presence of a

metal catalyst and found good yield of CO at high temperatures. He similarly found the SDC

showed higher yields than YSZ.

These increases in CO yield may be caused by the high number of oxygen vacancy sites

found using SDC. In contrast to YSZ (0.074 oxygen vacancy per cation), SDC (0.1 oxygen

vacancy per cation) has over 1.5 times the stoichiometric oxygen vacancies. Nonetheless, both

supports may participate actively in the reaction.

Both supports were also found to be stable at high temperatures when CuxNi1-x catalysts

of x = 50 and 20 were used. The Cu rich catalysts showed signs of deactivation over time. In

addition, there was no CH4 observed during the experiments. A Cu rich alloy found on the

surface of the catalyst may cause the absence of CH4. A Ni rich alloy combined with the

presence of Ni in the Cu rich surface may be the main contributor to the stability of the catalysts

observed at the high operating temperatures.

115

A.4 References







reverse water gas shift reaction over supported cu-ni ... · reverse water gas shift reaction over...

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